Preheat your oven to 375°F and grab a 10-inch oven-safe skillet. While it's heating up, whisk together the eggs and milk in a large bowl until everything's combined—don't stress about getting it perfectly smooth. This takes about 2 minutes tops and sets you up for success with your spring herb frittata recipe.

Heat the olive oil in your skillet over medium heat for about a minute until it shimmers slightly. Add the diced onion and sauté for 2-3 minutes until it softens and turns a little translucent. You'll know it's ready when your kitchen smells absolutely amazing (that's my favorite cooking signal).

Toss in the asparagus and fresh spinach now, cooking for another 2 minutes until the spinach wilts down. Don't panic if it looks like a ton of spinach at first—it shrinks way down when it hits the heat. Stir in the diced tomatoes too and let everything get cozy together in the pan.

Pour your egg and milk mixture directly over all those beautiful vegetables, making sure everything gets distributed evenly. Sprinkle the crumbled feta cheese over top, then scatter your fresh parsley and chives across the surface. Let it cook on the stovetop for 2 minutes until you see the edges start to set—don't mess with it during this time.

Transfer that skillet straight to your preheated oven and bake for 18-20 minutes until the center's set but still just slightly jiggly in the very middle. I always open the oven door and give it a gentle shake around minute 16 to check how it's coming along. You're looking for that golden-brown top that tells you it's perfectly done.

Take the skillet out of the oven and let it rest on the counter for about 3 minutes before you start cutting into it. This cooling time lets everything firm up just a little bit and makes it way easier to slice cleanly. I learned this the hard way after serving a wobbly mess to dinner guests once.

Run a thin spatula around the edges to loosen it from the pan, then slice it into 6 wedges right in the skillet. Serve it warm or at room temperature—this spring herb frittata recipe honestly tastes excellent either way, which is why it's perfect for meal prep situations.