Toss the sliced onion, cabbage, rice vinegar, 1/2 teaspoon salt and a few grinds of pepper in a large bowl.
Mix the salmon with the diced onion, eggs, panko, 1/4 cup dill, 2 tablespoons crème fraîche, 1/2 teaspoon salt and a few grinds of pepper in a medium bowl. Form into four 1/2-inch-thick patties. Mix 1/4 cup creme fraiche with the remaining 1 tablespoon dill in a small bowl.
Heat 2 tablespoons olive oil in a large nonstick skillet over high heat. Add the salmon patties and cook until browned and firm, 2 to 3 minutes per side.
Add the apple to the cabbage mixture. Fold in the remaining 1/4 cup creme fraiche and 2 tablespoons olive oil; season with salt and pepper. Serve the salmon burgers on the buns; top with the dill creme fraiche. Serve with the slaw.