In a medium-sized bowl, whisk together the eggs, milk, mustard, garlic salt, and salt and pepper.
Lightly grease the bottom of your slow cooker or spray with cooking spray.
Place half the potatoes on the bottom. Layer with half the bacon, chopped onion, mushrooms, broccoli, and cheese. Add the remaining potatoes, then top with the rest of the veggies, bacon, and cheese.
Pour egg mixture on top. Cover and cook on Low for 4 hours.