Heat oven to 350° F. Lightly spray 6 cups in a muffin tin with olive oil spray.: The oven should feel preheated and steady, providing even dry heat so the phyllo crisps without burning. You will notice the air in the oven shifting from cool to gently warm, and when the tin feels room temperature to the touch and the oven holds heat, you are ready. Use a light, even coating of spray to prevent sticking while keeping the shells crisp. A common mistake is over spraying, which can cause the phyllo to become greasy rather than flaky; use just enough to coat the surface.
Snap off and discard the woody end of the asparagus.: As you handle a spear, the natural breaking point tells you where the tender flesh begins, and snapping produces a clean break with a faint vegetal scent. Discarding the woody ends improves texture, preventing stringiness in the finished tartlet. If you leave tough ends in, the mouthfeel will be unpleasant and uneven; always check each spear before slicing.
Cut 1 1/2-inch long pieces from the tip end. Cut the remaining parts of the stalks into 1-inch pieces.: The tips provide a visual crown for each tartlet while the smaller pieces fill the custard with tender bites. As you slice, you'll see vibrant green contrast with paler stalk interiors, and that color translates to freshness after cooking. Uniformity is key here; mismatched sizes will lead to some pieces overcooking while others remain underdone.
Spray nonstick pan with olive oil spray over medium high heat. Add asparagus and garlic and sauté, stirring, for 2 minutes.: When the pan hits temperature, you should hear a quick sizzle as vegetables meet the surface, and garlic will quickly perfume the air. This high energy contact sears and concentrates flavor without fully softening the asparagus . Stirring keeps everything moving and prevents bits from sticking. Avoid letting the garlic brown too much, as it turns bitter; two minutes of attentive stirring is usually ideal.
Stir in the lemon juice and 1 tablespoon water. Reduce heat to low, cover and cook for 1 minute more.: Adding acid and a splash of water creates steam, and lowering the heat lets the asparagus finish cooking gently. You will notice a brief hiss and a deeper green emerge as chlorophyll brightens. This short steaming step preserves tenderness while integrating flavors. Rushing this step or keeping heat high can over soften the spears and dilute their fresh character.
Remove asparagus pieces to a plate using a slotted spoon and reserve the pan juices and garlic.: Lifting the vegetable with a slotted spoon leaves behind the concentrated, flavorful juices that will flavor the custard. The juices carry the savory, garlicky notes that tie the filling together. Resist the temptation to pour everything into the plate, because those pan juices are seasoning gold and you want them in the mixture, not lost on the pan.
Place a phyllo sheet on a clean work surface, lightly spray with oil and top with another phyllo sheet. Repeat with two more sheets and oil.: As you layer, the sheets should feel cool and pillowy, and a light coating of spray helps the layers adhere and brown uniformly. Working quickly prevents dryness, since exposed phyllo will crisp prematurely and crack. Keep unused sheets under a damp towel to avoid brittleness; dried sheets tear when you try to shape them.
Cut the phyllo into 4-inch squares. Fit one square each in the prepared muffin cups.: When you cut and shape the squares, they should fold into the cup with gentle, clean pleats and form a shallow shell. The edges will become crisp and slightly blistered in the oven, creating that satisfying crunch. If sheets crack while fitting, they were too dry; cover and rehydrate slightly with a spritz before trying again.
Evenly distribute the 1-inch asparagus pieces and feta among the muffin cups.: Place the small asparagus bits and crumbled feta so each cup gets a balanced portion, which ensures flavor consistency across tartlets. You should see an attractive mix of green spears and white crumbles. Overfilling will cause spillover and uneven baking, so keep portions modest for best results.
Whisk half and half, egg, dill, and reserved pan juices together. Evenly divide among the phyllo cups (about 3 1/2 tablespoons in each).: The custard should look glossy and smooth after whisking, with herb flecks distributed evenly. Pouring a consistent amount into each cup helps them set uniformly and produce the same delicate texture. If the custard is lumpy or uneven, whisk more thoroughly; a poorly mixed custard will bake inconsistently and create textural surprises.
Place two asparagus tips on each cup and bake for 30 minutes until custard is set and phyllo is golden.: While baking you will see the edges of the phyllo deepen to a warm golden brown and the custard lose its wet sheen, setting to a slightly puffed, matte surface. The aroma will shift from raw vegetables to toasty, buttery notes and tangy feta . A common pitfall is under baking causing a wobbly center; ensure the custard does not jiggle when the tin is gently nudged. Let the tartlets cool a bit so the custard firms up fully before removing them from the tin.