Saute the onion in olive oil and butter on medium heat, for about 3-4 minutes.: You will hear a gentle sizzle as the onion hits the warm pan and the aroma will shift from sharp to sweet as the pieces soften, becoming translucent around the edges. The hot fat carries flavor and prevents sticking, creating a silky base for the next ingredients. If the pan seems to spit loudly, lower the heat slightly to avoid browning too quickly, which would lead to bitter notes. A common mistake is rushing this step; undercooked onion leaves a raw bite that upsets the balance of the filling.
Stir in garlic and spinach and saute about another 5 minutes, until there is no liquid remaining. Add salt and pepper to taste.: As you add the minced garlic , its scent will bloom nearly instantly, fragrant and nutty. The spinach will collapse and steam out moisture, and you should continue cooking until the liquid evaporates so the filling does not make the shells soggy. You will see glossy greens reduced to a compact pile, and the sizzling will quiet down as water escapes. If the mixture seems watery after this time, transfer it to a colander and press gently to remove excess water, because failing to do so commonly yields soggy shells.
Transfer spinach onto the plate, and place in a cold place for a few minutes. Meanwhile, prepare the egg mixture by whisking the rest of the ingredients together.: When you move the hot spinach to a plate and let it cool, the heat stops further cooking and keeps the filling texture tender. The cooling step prevents the eggs from scrambling when combined, which would ruin the creamy binding. Whisk together the eggs , feta cheese , Parmesan cheese , chopped dill , and a pinch of salt and pepper until cohesive and slightly frothy, which helps the mixture set evenly in the oven. Rushing to combine while the greens are too hot can lead to curdled eggs, so patience here avoids that common error.
Add spinach to the egg mixture. Stir.: Once the spinach has cooled, fold it gently into the egg and cheese blend so the leaves remain tender and not mashed. You will notice the filling take on a speckled appearance from the cheeses and bits of dill , and the mixture should feel moist but not soupy. This step builds the final texture, where the binding action of the eggs and the salt from the cheeses harmonize. If the filling feels too loose, give it a final gentle press in a sieve to remove residual moisture, because overly wet filling is the main reason shells can become limp.
Divide the filling between the 15 Fillo shells. Bake at 350 F for 15 minutes. Let cool slightly before serving.: Spoon the prepared filling into each preformed fillo shell, filling them generously but not overflowing, so the tops bake evenly. You will see small mounds of filling that hold their shape, and as they bake the edges of the shells will take on color while the center sets. Aim to distribute the mixture evenly so each bite is balanced in cheese, herb, and green. A common slip up is overfilling, which causes spillage and uneven baking, so keep the amount moderate per shell.
Bake at 350 F for 15 minutes: Slide the tray into a preheated oven set to 350 F , and watch for a golden, crunchy rim on the shells and slight browning on the cheese. The kitchen will fill with a warm, savory aroma that signals the ingredients are melding. The centers should feel set but still tender when you press lightly with a fingertip using a clean utensil. If the tops brown too quickly, tent loosely with foil to allow the centers to finish cooking without burning the edges, because uneven heat exposure can cause the shells to char before the filling sets.
Let cool slightly before serving: After removing the tray, allow the bites to rest for a few minutes so the filling firms up and is easier to pick up. You'll notice the cheese settle and the steam diminish, making the texture more pleasant and preventing burns. This pause also helps flavors meld further and makes the bite more cohesive. Serving them piping hot is tempting, but cutting into them too soon can produce a loose filling, so patience ensures a tidy presentation.