Preheat oven to 375° F. Spray 13-by-9-inch baking dish with nonstick cooking spray.
In large bowl, whisk eggs. In large skillet over medium heat, heat oil. Add mushrooms, onion and bell pepper; cook 2 minutes, stirring occasionally. Add spinach; cook 3 minutes, or until spinach wilts, stirring occasionally. Stir milk, salt, pepper and vegetables into eggs; transfer to prepared dish.
Bake 25 minutes, or until set and internal temperature reaches 160° F; cool and cut into eight equal pieces.
Assemble sandwiches with muffins, sausage patties and egg mixture; wrap tightly with plastic wrap and freeze up to 2 weeks.
Unwrap one sandwich; wrap loosely with paper towel. Heat in microwave oven on high 2 1/2-3 minutes, or until completely heated through and internal temperature of sausage and egg mixture reaches 165° F.
We provide nutritional information but these figures should be considered estimates as they are not calculated by a registered dietitian. Please consult a medical professional for any specific nutrition, diet, or allergy advice.