Clean out the oysters and place them one by one in a big stockpot. Pour in the beer, as well as enough water to cover oysters completely. Add 2 cloves garlic, seasoned salt, and peppercorns to the pot and get the whole thing boiling. Take the pot off the heat, drain it out, and let it cool.

When the oysters have cooled off, break off and bin the top shells, then arrange them on a baking sheet. Turn the oven up to 425 F.

While the oven's heating up, melt the butter in a saucepan over medium heat. Cook the onion and garlic in there until they turn soft. Reduce the heat to low, and stir in the spinach, Monterey Jack, fontina, and mozzarella. Continue cooking, stirring frequently, until the cheese melts. Stir in the milk, and season with salt and pepper. Spoon this sauce over each oyster. Don't add too much, you want enough to just fill the shell. When the shells are full, sprinkle them with bread crumbs.

Bake the oysters in the preheated oven until golden and bubbly, approximately 8 to 10 minutes.