Prep: 25 mins
| Cook: 35 mins
| Extra time: 10 mins, resting
Preheat oven to 350 degrees F and coat a 9x13-inch baking dish with nonstick cooking spray.
Pour oil into a large skillet and set over medium heat. Cook onion in hot oil for about 8 minutes, until softened, then stir in garlic and cook for 1 more minute. Add spinach to the skillet and season with salt and pepper. Set aside.
Meanwhile, brown bacon in a small skillet over medium heat. Drain on paper towels, pouring any rendered fat into the skillet with onion and spinach.
In a mixing bowl, beat eggs with milk; stir in 1/2 cup fontina and 1/2 cup Cheddar, Worcestershire, mustard, and nutmeg.
Pour the hash brown potatoes into the prepared baking dish and top with the spinach mixture. Sprinkle with browned bacon, then pour the egg cheese mixture over the casserole. Top with the rest of the cheese.
Bake for 35 minutes until eggs are set. Remove from oven; allow the casserole to rest for 10 minutes, then slice and serve.