Dredge salmon fillets in flour. Place a large frying pan over medium heat, add the butter and oil, stir to combine when the butter has melted. Sauté the salmon until golden on both sides and cooked through, turn off the heat. Drizzle the salmon with soy sauce while the salmon is still in the pan, gently shake the pan to mix soy sauce with the oil and butter. Add wasabi to the warmed mashed potatoes, and divide the potatoes into 4 bowls. Place 1 salmon fillet onto each bowl.