Cut a large sheet of aluminum foil into four 8-inch square pieces. Place 1 salmon fillet on each piece of foil; season with salt and pepper. Drizzle olive oil and balsamic vinegar over fillets. Sprinkle dill fronds on top. Place 1 lemon slice over each fillet.
Lift up ends of aluminum foil and fold together to create packets around salmon fillets. Transfer packets to a baking sheet.
Bake in the preheated oven until salmon flakes easily with a fork, about 20 minutes.
Combine creme fraiche, lemon juice, salt, and pepper in a small saucepan over low heat. Stir until sauce is heated through, 3 to 5 minutes.
Open foil packets carefully. Pour sauce over salmon.