Preheat the oven to 425°F and arrange the oysters on baking sheets.
Cook bacon in a large skillet over medium heat until crisp. When bacon is done cooking, let it drain on paper towels before transferring it to a cutting board and roughly chopping it. Set aside until ready to use.
Melt the butter in a saucepan over medium heat. Cook onion and garlic in the butter until softened and slightly browned. Reduce heat to low, and stir in the spinach, cheddar, fontina and mozzarella. Cook until the cheese melts, stirring frequently. Stir in the milk and season with salt and pepper.
Top each oyster with desired amount of chopped bacon. Then spoon the sauce over each oyster, using just enough to fill the shell. Top each oyster with a sprinkle of fontina.
Bake oysters until the cheese is golden and bubbly, about 8-10 minutes.