1. Prepare the Egg Mixture: In a medium bowl, whisk together the 8 large eggs, 1/4 cup milk, 1/2 teaspoon salt, and 1/4 teaspoon black pepper until the mixture is well combined and slightly frothy. Set this aside.

2. Preheat Oven and Saute Aromatics: Preheat your oven to 375 F (190 C). Heat 1 tablespoon of olive oil in your oven-safe, non-stick skillet over medium heat. Add the finely diced onion and cook for 3-4 minutes, stirring occasionally, until it becomes soft and translucent.

3. Add Asparagus and Garlic: Stir in the 1-inch pieces of asparagus and minced garlic into the skillet with the softened onions. Continue to cook for another 3-5 minutes, stirring frequently, until the asparagus is tender-crisp and bright green. You want it cooked but still with a slight bite.

4. Pour in Egg Mixture: Evenly spread the cooked asparagus and onion mixture across the bottom of the skillet. Pour the whisked egg mixture over the vegetables in the skillet. Gently shake the pan a little to ensure the eggs settle around the vegetables.

5. Add Mozzarella and Initial Cook: Sprinkle the 1/2 cup of shredded mozzarella cheese evenly over the top of the egg mixture. Let the frittata cook on the stovetop for 2-3 minutes without stirring, until the edges just begin to set and pull away slightly from the pan. This helps create a nice crust.

6. Finish in the Oven: Carefully transfer the skillet to the preheated oven. Bake for 12-15 minutes, or until the frittata is set in the center, puffed up, and the cheese is melted and lightly golden. A knife inserted into the center should come out clean.

7. Rest and Serve: Once baked, carefully remove the frittata from the oven. Let it rest in the skillet for 5 minutes before slicing it into wedges. This resting period allows the frittata to firm up slightly and makes for cleaner slices. Serve warm.