Grease the slow cooker: Begin by greasing your 4-quart slow cooker with cooking spray or a light layer of oil to prevent sticking.
Layer the base: Evenly layer 1/3 of the 32 oz bag of frozen hash browns on the bottom of the slow cooker. Top this with half of the 1 lb of precooked turkey bacon or chicken, followed by half of the 1/2 chopped onion and optional 1 chopped green pepper. Season with salt and pepper to taste, then sprinkle 1/2 cup of the grated cheddar cheese over the top.
Repeat the layers: Create another layer with the remaining hash browns, turkey bacon or chicken, onion, green pepper (if using), and another sprinkle of salt and pepper, followed by another 1/2 cup of cheese. Finish off with the last 1/3 of hash browns, reserving the remaining cheese for later.
Prepare the egg mixture: In a large mixing bowl, whisk together the 12 eggs and 3/4 cup of milk until fully combined. Pour this mixture over the layered ingredients in the slow cooker, ensuring even distribution.
Top with cheese: Finally, sprinkle the remaining 1/2 cup of grated cheddar cheese on top of the egg mixture for a melty topping.
Cook: Cover the slow cooker and set it to cook on low for 6-8 hours. The breakfast is done when the eggs are set and the cheese is bubbling. Opt for serving warm and garnishing with chopped herbs if desired.