Heat a skillet over medium heat. Cook sausage until browned, about 5 minutes. Transfer to a bowl.
Heat oil in the same skillet over medium-high heat. Saute mushrooms, onion, and red bell pepper with 1/2 the salt and pepper until vegetables are tender, about 5 minutes.
Preheat the oven to 375 degrees F (190 degrees C). Line a 12-cup muffin tin with paper liners.
Whisk eggs with remaining salt and pepper in a large bowl. Add shredded Mexican cheese blend, spinach, milk, chives, and the sauteed sausage and vegetables.
Fill each muffin cup no more than 3/4 full with the egg mixture using an ice cream scoop.
Bake in the preheated oven until golden brown, 20 to 25 minutes.
Muffins can be stored in an airtight container in the fridge for up to 3 to 4 days. Wrap the rest and toss in freezer for a quick weekday breakfast (just microwave to reheat).
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