Place cheesecloth on a flat surface. Heat peppercorns, coriander seeds, dried crushed red pepper, mustard seeds, dill seeds, celery seeds, and crushed bay leaves in a hot skillet over medium-high heat, stirring constantly, 1 to 2 minutes or until fragrant. Cool 10 minutes. Stir in kosher salt. Divide mixture between 2 cheesecloth stacks. Gather edges of cheesecloth together; tie with kitchen string. Store at room temperature up to 2 weeks. For gifting, package each sachet with 3 lemons, 2 yellow onions, and 2 garlic bulbs.
TO MAKE BOILED SHRIMP: Bring 6 cups water; 3 lemons, sliced; 2 yellow onions, sliced; 2 garlic bulbs, and 1 Shrimp-and-Crab Boil Sachet to a boil in a large Dutch oven over medium-high heat; boil 30 minutes. Add 3 lb. unpeeled, large raw shrimp; cover and remove from heat. Let stand 10 minutes or just until shrimp turn pink. Discard sachet, and serve shrimp boil with Carolina Mustard Sauce. Makes 6 servings. Hands-on 15 min.; Total 1 hour, 5 min.