Hash Brown Breakfast Casserole

Name: Hash Brown Breakfast Casserole
Ingredients: 1 20 oz bag frozen shredded hash browns (about 4-5 cups); 3 teaspoons olive or avocado oil (divided); 1 cup yellow onion (chopped); 2 cloves garlic (minced ); 1 red bell pepper (chopped); 1 8 oz package mushrooms (chopped); 1 5 oz bag of baby spinach; 1 teaspoon salt (plus more to taste); ½ teaspoon ground black pepper (plus more to taste); 10 eggs; ½ cup non-dairy milk (I used unsweetened almond milk); 1 ½ cups shredded sharp cheddar cheese