Preheat the Oven: Start by preheating your oven to 350°F (175°C). This will ensure that the frittata cooks evenly.
Sauté the Asparagus: In an oven-safe skillet, heat olive oil over medium heat. Add the chopped onion and asparagus, sautéing for about 5 minutes until they’re tender but still vibrant green.
Prepare the Egg Mixture: In a mixing bowl, whisk together the eggs, milk, grated Parmesan cheese, salt, and pepper until well combined. You want a fluffy texture here, so whisk vigorously!
Combine Ingredients: Pour the egg mixture over the sautéed asparagus and onion in the skillet. Stir gently to distribute the vegetables evenly.
Cook on the Stove: Allow the frittata to cook on the stovetop for about 5-7 minutes, or until the edges start to set.
Transfer to the Oven: Carefully transfer the skillet to the preheated oven. Bake for an additional 10-15 minutes, or until the frittata is set in the center and lightly golden on top.
Let it Stand: Once baked, remove the skillet from the oven and let the frittata stand for about 5 minutes. This helps it set further and makes it easier to slice.
Serve: Slice the frittata into wedges, garnish with fresh herbs if desired, and serve warm or at room temperature.