1. Prepare Your Vegetables: Start by trimming the tough, woody ends off your asparagus. The easiest way to do this is to hold an asparagus spear by both ends and bend it; it will naturally snap where the tough part begins. Then, cut the tender asparagus into 1-inch pieces. If using fresh peas, give them a quick rinse. Finely chop your yellow onion.

2. Saute the Aromatics and Asparagus: Heat the olive oil in an oven-safe, non-stick skillet (about 8-10 inches) over medium heat. Add the chopped onion and cook for 2-3 minutes until it softens and becomes translucent. Add the asparagus pieces to the skillet and cook for another 3-5 minutes, stirring occasionally, until they are tender-crisp and bright green.

3. Add the Peas: Stir in the peas (fresh or frozen) and cook for just 1-2 minutes more, until they are heated through. You want the vegetables to be cooked but still have a slight bite. Remove the skillet from the heat for a moment.

4. Whisk the Egg Mixture: In a medium bowl, whisk together the eggs, milk, salt, and black pepper until well combined and slightly frothy. If you’re using Parmesan cheese, you can stir about half of it into the egg mixture now, or save it all for sprinkling on top later.

5. Combine and Cook on Stovetop: Pour the egg mixture evenly over the vegetables in the skillet. Return the skillet to medium-low heat. Cook on the stovetop for about 5-7 minutes, without stirring, until the edges of the frittata begin to set and pull away from the sides of the pan, and the center is mostly set but still looks a little wet.

6. Finish in the Oven: If using Parmesan cheese, sprinkle the remaining cheese over the top of the frittata. Transfer the skillet to a preheated oven (set to 350 F (175 C)). Bake for 8-12 minutes, or until the frittata is puffed, golden brown on top, and cooked through. A knife inserted into the center should come out clean.

7. Serve and Garnish: Carefully remove the frittata from the oven. Let it rest in the skillet for a few minutes before sliding it onto a cutting board or serving platter. Garnish with fresh chopped chives, if desired. Slice into wedges and serve warm.