Preheat oven to 350˚F.
Crumble sausage into bite-sized pieces into a large skillet. Cook the sausage over medium heat until it is no longer pink, approximately 5-7 minutes.
Remove from heat and carefully drain excess fat.
Crack eggs into a large bowl and beat thoroughly with a fork.
Add cooked sausage, Portobello mushrooms, green onion, and one half of the cheese blend.
Season with salt and black pepper, to taste.
Vigorously stir with a fork to combine.
Spray both muffin tins thoroughly with non-stick cooking spray and scoop ¼ cup of egg mixture into the opening. (Batch yields approximately 16 muffins).
When finished, top each cup with equal amounts of the remaining Mexican-blend cheese.
Place muffin tins in preheated oven and bake for 20–25 minutes or until eggs are set.
Remove from oven and cool slightly before removing from pan.
Serve immediately or place in a freezer-safe container to freeze for later use.