Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan.

Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Transfer bacon to a paper towel-lined plate and drain grease from the skillet. Chop bacon; set aside.

Sauté bell pepper and onion in the same skillet over medium-high heat until tender, 5 to 10 minutes. Spread spinach into the bottom of the prepared baking pan; top with bacon and bell pepper mixture.

Beat eggs in a bowl with a fork. Add milk, garlic salt, and pepper to eggs and beat until well combined. Pour egg mixture over bacon mixture and sprinkle with Monterey Jack cheese and Asiago cheese.

Bake in the preheated oven until a knife inserted into the center comes out clean, 35 to 45 minutes.