Preheat the oven: Set your oven to 375°F (190°C).
Use an oven-safe 10-inch skillet, preferably cast iron or stainless steel.
Prepare the egg mixture: In a large bowl, whisk the eggs, milk, salt, and pepper until smooth and slightly frothy. Stir in the herbs and lemon zest. Set aside.
Sauté the aromatics: Heat the olive oil in the skillet over medium.
Add the leeks with a pinch of salt and cook 3–4 minutes until softened. Stir in the garlic and cook 30 seconds until fragrant.
Cook the veggies: Add asparagus pieces and cook 3–4 minutes until they turn bright green and just tender. Stir in peas and spinach; cook 1–2 minutes until the spinach wilts.
Taste and season lightly.
Add the cheese: Scatter half the feta over the vegetables in the skillet. Give the egg mixture one more whisk and pour it evenly over everything. Gently shake the pan to distribute.
Top and set: Sprinkle the remaining feta and a pinch of red pepper flakes over the top.
Cook on the stovetop for 1–2 minutes without stirring, just until the edges begin to set.
Bake: Transfer the skillet to the oven and bake 10–14 minutes, until the center is just set and slightly jiggly. The top should look puffed and lightly golden.
Rest and serve: Let the frittata rest 5 minutes. Slice into wedges.
Finish with extra herbs or a squeeze of lemon if you like.