Preheat the oven broiler. Line a baking sheet with parchment paper.

Heat 1 teaspoon vegetable oil in a skillet over medium heat. Stir in red onion and green bell pepper. Cook and stir until tender, 5 to 8 minutes. Transfer to a bowl, and mix with mayonnaise, Dijon mustard, Worcestershire sauce, and seafood seasoning.

Melt 1 tablespoon butter in a skillet over medium heat. Place salmon and shrimp in the skillet, and cook until slightly opaque and coated with butter, about 2 minutes. Transfer to a large bowl.

Place panko bread crumbs and 3 tablespoons butter in a microwave-safe bowl. Cook on High until butter is melted, about 20 seconds. Mix crumbs and butter, and season with garlic salt.

Add the mayonnaise mixture 1 tablespoon at a time to moisten the salmon and shrimp mixture. Mix breadcrumb mixture, 1 tablespoon at a time, into the bowl with the salmon and shrimp mixture, just until the ingredients are easily formed into cakes. Form 6 4-inch cakes, and place on the prepared baking sheet.

Broil salmon and shrimp cakes in the preheated oven until golden brown, 7 to 10 minutes, turning once.