Preheat the oven to 375 degrees F. Spray a nonstick 10-inch cast iron pan with oil. Set aside.
Gently place the jumbo lump crab onto a clean surface and inspect for any remaining pieces of shell or cartilage. Remove any fragments left behind but take care not to break the delicate lumps. Gently pat them dry with a paper towel.
Combine mayo, breadcrumbs, parmesan, dijon mustard, fresh parsley, lemon juice, garlic paste, Worcestershire sauce, old bay seasoning, hot sauce, and lemon zest in a large bowl. Mix everything together until combined, then add the crabmeat. Expert tip:When mixing the stuffing, handle it gently to avoid breaking up the lumps, so each bite is full of that fresh crab flavor. Place mixture into the refrigerator while preparing the shrimp.
Rinse the shrimp in cold water, remove the shells (if applicable), and use a sharp pairing knife to cut along the back from top to tail, being careful not to cut all the way through. Press it open so it lies flat, and remove any veins or shell fragments before cooking. If you bought deveined shrimp, you don't have to worry about removing the vein.
Add shrimp to a large mixing bowl and season with old bay, garlic powder, onion powder, herbs de provence, and salt and black pepper. Toss to coat, ensuring every shrimp is covered in the seasoning. Take a shrimp and pack the shrimp with the crab mixture (about 1-2 tbsps, per your personal preference and depending on the size of the shrimp you use). Place the shrimp, butterfly side down onto the cast iron pan.
Use a pastry brush to lather the shrimp in the about 1/2 of the garlic butter. Bake for 15 minutes, then broil for 2-3 minutes, until the filling develops a golden brown crust. Coat the shrimp in the remaining garlic butter and sprinkle with red pepper flakes and a squeeze of fresh lemon juice. Serve with additional lemon wedges.
Notes
Keep an Eye on Cooking Time: Shrimp cook quickly, so watch them closely. Overbaking can make the shrimp rubbery, so take them out as soon as they turn opaque and the stuffing is just cooked through. I bake mine for 15 mins, but every oven is different so keep an eye on them after the 10-12 minute mark. Butterfly the Shrimp Properly: Make a clean cut along the back of the shrimp without slicing all the way through. Opening them up gives you room for a generous amount of stuffing.Choose Quality Crab: Use the best jumbo lump crab meat and use little filler. This helps the sweet flavor of the crab really shine.Gentle Mixing: When combining your crab mixture, fold ingredients carefully so you don’t break up the lumps too much—those big chunks are the highlight of the dish.If using fresh shrimp: be sure to use 24 hours after buying to prevent spoilage.