In a large pot of water, combine the crab boil, 1 teaspoon of salt, the quartered lemon, and 6 cups of water. Bring to a boil over high heat and boil for 5 minutes. Add the shrimp and cook until pink and cooked through, 1 to 2 minutes. Drain in a colander.
In a large bowl, combine the Creole mustard, remaining 1/2 teaspoon salt, black pepper, and red pepper flakes. Whisk in the vinegar to combine. In a steady stream, whisk in the oil until emulsified. Add the garlic and bay leaf. Place the shrimp in a glass baking dish and cover with the marinade. Toss with the onions and lemon slices to evenly distribute.
Cover tightly with plastic wrap and refrigerate for 12 to 24 hours before serving.
Preheat a grill.
Prepare pizza dough and roll out into 4 (6-inch) rounds. Brush the dough with oil and place on the grill. Cook for about 3 minutes per side, until cooked through. Chop arugula and toss with olive oil, lemon juice and salt and pepper. Place the arugula on top of the pizza dough and top with shrimp.
Yield: 2 pounds, 6 to 8 servings