Preheat oven to broil. Pour rock salt onto a rimmed baking sheet.
In a small saucepan, melt 1 tablespoon of butter over medium-high heat. Add garlic; cook until lightly browned, about 2 minutes. Add lemon juice, Worcestershire, Slap Ya Mama - Cajun Hot Sauce, Slap Ya Mama - Original Blend Cajun Seasoning, pepper, and kosher salt; cook for 2 minutes. Add beer; cook until reduced by half, 15 to 20 minutes. Add remaining 2 tablespoons butter, stirring until combined.
Arrange oysters on prepared pan. Divide sauce among oysters.
Bake until edges of oysters are curled, 5 to 10 minutes. Garnish with rosemary if desired!