Clean oysters, and place in a large stockpot. Pour in beer and enough water to cover oysters; add 2 cloves garlic, seasoned salt and peppercorns. Bring to a boil. Remove from heat, drain and cool.
Once oysters are cooled, break off and discard the top shell. Arrange the oysters on a baking tray. Preheat oven to 220 degrees C.
Melt butter in a saucepan over medium heat. Cook onion and garlic in butter until soft. Reduce heat to low and stir in spinach and cheese. Cook until cheese melts, stirring frequently. Stir in the milk and season with salt and pepper. Spoon sauce over each oyster, just filling the shell. Sprinkle with bread crumbs.
Bake until golden and bubbly, approximately 8 to 10 minutes.