Preheat the oven to 400°F. In a medium or large cast-iron skillet, heat the oil over medium-high heat. Once hot, add the Italian sausage. Use a wooden spoon or meat chopper to break up the meat. Sauté until the meat is lightly brown on all sides.
Stir in the garlic, onions, peppers, and asparagus, and sauté until the vegetables are soft and fragrant, about 2 to 3 minutes.
In a medium bowl, beat the eggs and almond milk (or DF heavy cream) together. Pour the eggs over the vegetables. Immediately sprinkle the shredded cheese evenly over the eggs.
Allow the cheese to settle into the egg mixture for one minute, then carefully transfer the hot skillet to the middle rack of the oven.
Bake at 400°F for 10 to 15 minutes, or until it's fully baked. The center will be set and no longer runny or jiggly when it's done. Allow the frittata to cool in the pan at room temperature for 5 minutes.
Notes
STORAGE: If using FoodSaver Meal Prep Bags, allow the frittata to cool completely. Then cut it into 6 to 8 slices. Use the Meal Prep Bags to vacuum seal each slice for freshness, then freeze. When you're ready to eat, take out a slice and reheat the frittata slice directly from the freezer in the microwave.