Combine mayonnaise, mustard, egg (if using), and Old Bay Seasoning in a mixing bowl.
Add diced onion, celery, and crushed saltines to the bowl with the wet ingredients and mix to incorporate.
Add crabmeat to the bowl, breaking up large lumps of crabmeat and removing any stray shell pieces. Fold the crab into the cracker mixture just until combined. I like to leave some small clumps of crab for visual appeal.
Set crab mixture aside while preparing salmon.
Salmon
Preheat oven to 400°F.
Pat the salmon dry and place it in a glass baking dish or on a baking sheet lined with parchment paper. Brush with olive oil and generously season with salt and pepper.
Use a small sharp knife to cut a slit down the center of the salmon. Don't cut through the fillet entirely, and leave the fish uncut at each end. Use your fingers or a fork to gently spread the opening apart.
Fill the center of each salmon fillet with about 2 Tablespoons of crab stuffing, gently pressing some down into the bottom of the slit and allowing the rest to mound over the top.
Drizzle melted butter over the top of each salmon fillet.
Bake uncovered for 15-20 minutes in a 400°F oven or until cooked through and the internal temperature reaches about 135°F. See notes on cooking temperatures below.
Allow crab stuffed salmon to rest for about 5 minutes so the juices redistribute. Serve with lemon wedges or optional lemon butter sauce (see instructions below.)
Notes
See more expert tips and step-by-step pictures above!Air Fryer InstructionsPreheat the air fryer to 390°F. Spritz the basket with olive oil before placing the salmon inside and cook for 10-12 minutes or until the internal temperature reaches 135°F.Storage
Store in an airtight container in the refrigerator for 1-2 days. Reheat in the oven at low heat (about 275°F) for about 15 minutes.
Crab stuffed salmon can be frozen before or after you cook it in an airtight container. For the best result, enjoy within 2 months of freezing. Defrost in the refrigerator overnight and then cook as usual.
Lemon Butter SauceMelt ¼ cup butter in a small saucepan over low heat. If using unsalted butter, add salt to taste or about ¼ teaspoon. Remove from heat once it has melted and stir it with 2 Tablespoons of lemon juice. You can add chopped fresh herbs or red pepper flakes for a twist on the basic recipe.Cooking TemperatureCooking salmon quickly at a high temperature allows it to cook and retain moisture so it's not dried out. The temperature will continue to rise once it's out of the oven. Allow it to rest for about 5 minutes so that the juices redistribute.Note: The FDA recommends cooking seafood to an internal temperature of 145°F. However, most professional chefs recommend taking it out a little earlier.