Step 1

Arrange a rack in the middle of the oven and heat the oven to 350°F. Line a rimmed baking sheet with parchment paper.

Step 2

Pick and finely chop the leaves from 1/2 bunch fresh parsley leaves until you have about 1/2 cup. Place 10 Ritz crackers in a small zip-top bag, press out the air, and seal the bag. Crush the crackers a rolling pin or bottom of a saucepan into crumbs (about 1/3 cup).

Step 3

Pat 4 skinless salmon fillets dry with paper towels, then place on the baking sheet. Starting 1 inch from one end, cut a slit down the center of each fillet lengthwise, cutting all the way through to the bottom and stopping 1 inch from the other end to make a pocket. Season the salmon with 2 teaspoons of the Old Bay seasoning and 1 tablespoon olive oil, then rub to distribute evenly on all sides of the salmon.

Step 4

Squeeze the juice of 1/2 medium lemon into a medium bowl until you have 1 tablespoon. Add the remaining 2 teaspoons Old Bay seasoning, 2 tablespoons mayonnaise, 1 tablespoon whole grain mustard, and 1/2 teaspoon black pepper. Stir until well combined. Drain 8 ounces jumbo lump crab meat (about 2 cups). Add the crab, cracker crumbs, and half of the parsley. Fold until well combined. Stuff the filling into the salmon pockets (about 1/2 cup each).

Step 5

Bake until the salmon registers 120º F on an instant-read thermometer for medium rare. Meanwhile, make the sauce.

Step 6

Cut 8 tablespoons unsalted butter into 4 pieces and place in a small saucepan. Melt over medium heat. (Alternatively, melt in the microwave in 10-second bursts.) Squeeze the juice from 1/2 medium lemon until you have 1 1/2 teaspoons.

Step 7

Place the lemon juice, 3 large egg yolks, 2 teaspoons Dijon mustard, 1/2 teaspoon whole grain mustard if using, 1/4 teaspoon kosher salt, and 1/8 teaspoon cayenne if using in a blender or tall liquid measuring cup if using an immersion blender. Blend on medium speed until combined, 5 to 10 seconds. With the blender on medium speed, slowly pour in the hot butter and blend until pale yellow and emulsified, 45 to 60 seconds. Serve the sauce over the salmon and garnish with the remaining parsley.