Place the bacon in a large, deep skillet, and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain the bacon slices on a paper towel-lined plate, then chop. Set aside. Season mahi mahi with garlic salt and pepper. Heat a skillet over medium heat. Cook mahi mahi until it flakes easily, 10 to 15 minutes. Remove from skillet and place each filet on a baking dish.
Preheat the oven's broiler and set the oven rack about 6 inches from the heat source.
While the fish is cooking, melt the butter in a separate skillet. Stir in the green onions, garlic, and spinach. Cook and stir until heated through, 2 to 3 minutes. Stir in the chopped bacon, hot pepper sauce, and evaporated milk. Bring to a low simmer, then add the Monterey Jack cheese a little at a time until fully melted. Stir in more evaporated milk, if needed, to create a sauce. Divide the spinach mixture evenly over each mahi mahi fillet. Sprinkle with Parmesan cheese.
Broil in the preheated oven until the cheese has melted and is lightly browned, 3 to 5 minutes more.