Crumble sausage with your hands into bite-sized pieces into a large skillet. Over medium-high heat, cook the sausage until it is no longer pink. Remove from heat and carefully drain excess fat.
Crack eggs into a large glass bowl and beat thoroughly with a fork. Add cooked sausage, Portobello mushrooms, green onion, ½ cheese blend, salt and pepper. Vigorously stir with a fork to blend all ingredients.
Spray both muffin tins thoroughly with non-stick cooking spray and scoop ¼ cup of egg mixture into each container. (You should have enough for approximately 16 muffins)
When finished, top each cup with equal amounts of the remaining Mexican-blend cheese.
Place in oven and bake for 20–25 minutes. Remove from oven and cool slightly before removing from pan. Serve immediately or place in freezer-safe container to freeze for later use.