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.
