Spinach + Mushroom Egg Muffins

Name: Spinach + Mushroom Egg Muffins
Cuisine: American
Ingredients: 1 Tbsp avocado oil or olive oil; 6 oz button mushrooms (thinly sliced); 1 medium yellow onion (small diced); 1 bell pepper (small diced); 3 fresh garlic cloves (pressed); 2 tsps dried oregano; 8 ozs fresh spinach (coarsely chopped); 8 large eggs; 1/2 cup milk of choice; 1/2 cup freshly shredded cheddar cheese; sea salt and ground pepper (to taste)