Preheat your oven to 500 degrees (or full broil). Using a large mixing bowl, whisk together the eggs, milk, dijon mustard and season with salt and pepper. Mix in the mozzarella cheese and set aside.
Using a cast iron skillet, or any large oven-safe skillet, heat the olive oil over medium-high heat. Add in the onions and mushrooms and sauté until tender, for about 5 minutes. Add in the baby spinach and blanched asparagus and cook until the spinach leaves become softened, for about 4 to 5 minutes.
Pour in the egg mixture, and turn the heat down to medium. Cook until set, for about 6 to 8 minutes. Transfer the skillet to the oven and broil until golden brown on top. Be sure to watch your frittata carefully. Remove from the oven and let cool for at least 5 to 7 minutes before topping with the spring salad mix. Add some sliced radishes if desired and salad dressing. Serve and enjoy!