Clean oysters, and place them in a large stockpot. Pour in beer and enough water to cover oysters; add 2 cloves of garlic, peppercorns, and seasoned salt. Bring to a boil. Remove from heat, drain, and cool.

Preheat the oven to 425 degrees F (220 degrees C).

Once oysters are cooled, break off and discard the top shell. Arrange oysters on a baking sheet.

Melt butter in a saucepan over medium heat. Cook onion and garlic in butter until soft. Reduce heat to low; stir in spinach, Monterey Jack, fontina, and mozzarella. Cook until cheese melts, stirring frequently. Stir in milk; season with salt and pepper. Spoon sauce over each oyster, just filling the shell. Sprinkle with bread crumbs.