In a 12 inch ovenproof skillet, melt butter and cook onions and celery in butter over medium heat until soft and translucent. Add the garlic and cook for 30 seconds. Stir in flour and cook for a minute.: When the butter hits the pan it should gently foam and release a warm, nutty aroma, not brown aggressively. As the onion and celery sweat, you will notice their scent deepen and a slight sweet steam rise, signaling they are softening. Use a wooden spoon to stir every 30 seconds to keep even contact with the pan, and aim for translucent, tender pieces rather than caramelized edges. This step builds the flavor foundation, softening fibrous vegetables and releasing sugars that balance the final sauce. A common mistake is turning the heat too high, which can make the vegetables brown too quickly and introduce bitter notes.
Slowly stir in chicken broth and milk. Add the salt, celery seed, poultry seasoning, and pepper. Simmer over medium-low heat until thick.: The moment you add minced garlic , the kitchen will fill with a sharp, aromatic burst that mellows rapidly, so time this carefully. After 30 seconds the raw edge will round off and it will begin to smell toasty, but if you let it sit longer it can scorch and turn acrid. Stir continuously and watch the pan closely, because garlic transitions quickly from fragrant to burnt. The purpose here is to layer in an aromatic lift without overpowering the base flavors.
Remove from heat and stir in chicken, carrots and peas.: When you incorporate the flour into the butter and vegetables, you are creating a roux that will thicken the sauce. The mixture should smell faintly nutty after about a minute, losing its raw flour scent. Stir constantly to prevent lumps and to toast the flour just enough to add depth. If you undercook it the final sauce can taste floury, while overcooking risks a dry, pasty texture, so keep the heat moderate and the motion steady.
Roll out pastry puff just a little thinner. Cut into a 4×4 square (16 pieces). Layer pieces on top of the chicken filling in any pattern you like. Whisk egg with a tablespoon of water and brush puff pastry with it.: As you add the chicken broth and then the milk , pour in a steady stream while whisking or stirring so the roux disperses evenly. You will hear a soft simmer as the liquid loosens the roux and the sauce begins to thicken. The aroma will deepen into a rich, savory scent and the texture should move from pasty to velvety. If you add the liquids too fast, lumps may form, so patience is the key. The goal is a smooth, clingy sauce that coats a spoon.
Bake in the preheated oven for 25 to 30 minutes on a baking sheet, or until pastry puff is golden brown and filling is bubbly. Cool for 10 minutes before serving.: Seasoning now lets the spices bloom into the warmed sauce, releasing their herbal and slightly nutty notes. Stir these in and let the mixture simmer on medium low until it develops body and a sheen. Taste carefully, because the chicken broth might already contain sodium, and adjust as needed. Adding too much salt early can be hard to correct, so season gradually and reassess after the filling cools slightly.
Simmer over medium-low heat until thick: As the mixture simmers, you will notice it reduce and take on a creamy thickness that clings to the back of a spoon, and tiny bubbles should break gently at the edges. This transformation concentrates flavor and ensures the filling will not be watery under the pastry. Keep the heat moderate so it does not boil vigorously, which can thin the sauce by breaking down starches. A common error is rushing this step, resulting in a runny filling under the pastry.
Remove from heat and stir in chicken, carrots and peas: Off the heat, fold in the diced rotisserie chicken and frozen vegetables so they warm through without overcooking. The residual heat will soften the peas and carrots while keeping their texture intact. Stir gently to distribute ingredients evenly, and notice how the mixture becomes more homogenous and visually appealing. If you stir too vigorously you can break up the chicken into shreds, which changes the mouthfeel.
Roll out pastry puff just a little thinner: On a lightly floured surface, ease the thawed puff pastry a touch thinner so it browns evenly and is easy to cut. You should feel a supple, cool dough that responds to gentle pressure. Rolling too thin makes it fragile and liable to soak up filling moisture, while too thick can bake unevenly. Keep it chilled enough to handle without sticking, and work quickly so the butter layers stay distinct for flakiness.
Cut into a 4×4 square (16 pieces): When you cut the pastry into a 4 by 4 grid, you create bite sized tiles that will puff individually and crisp beautifully. Use a sharp knife or pizza cutter for clean edges so the pieces rise uniformly. The visual pattern also helps steam escape in small channels, preventing sogginess. Dull blades can compress the layers and inhibit puffing.
Layer pieces on top of the chicken filling in any pattern you like: Arrange the pastry pieces over the filling in a single layer, leaving small gaps for steam. As the pastry browns, you will hear a faint popping and see edges lift, creating a mosaic of golden pockets. This playful layering lets you control coverage and steam release. Overlapping too much can trap moisture and prevent full crisping at the edges.
Whisk egg with a tablespoon of water and brush puff pastry with it: The egg wash should be smooth and glossy, and brushing it onto the pastry will give a deep, even golden color while helping pieces cling. Use a light, even coat and avoid heavy pooling. Excessive egg wash can run into seams and inhibit crisping, leading to a sticky finish rather than a shiny, crisp top.
Bake in the preheated oven for 25 to 30 minutes on a baking sheet, or until pastry puff is golden brown and filling is bubbly: During baking you will smell a rich, toasty aroma and see the pastry turn a beautiful deep gold while the filling bubbles at the edges. Baking on a sheet catches any spills and promotes even heat circulation. If the pastry browns too quickly on top, tent lightly with foil to prevent burning while the filling finishes. Underbaking leaves the center cool and the pastry pale, so ensure the filling is bubbling and the crust is golden.
Cool for 10 minutes before serving: Allow the skillet to rest so the filling settles and the steam dissipates, making scooping easier and safer. The heat will mellow and the texture will firm slightly, giving cleaner portions. Serving immediately risks burns and a soupy texture, while waiting too long can cool the filling excessively. Ten minutes is the sweet spot for serving with confidence.