Preheat oven to 400 ºF (200 ºC).
Pat the salmon dry with paper towel.
Spread 1 teaspoon (5 mL) of Dijon mustard on the fleshy side of each fillet. Lightly season with salt and pepper.
Peel, wash and grate the potatoes.
Stir in the minced onion into the grated potatoes.
Divide potatoes into 4 equal portions. Use your hands to squeeze excess moisture from the potato mixture, and press the mixture firmly and evenly onto the fleshy side of the fillet.
Heat oil in a large non-stick ovenproof pan.
Carefully place each fillet, potato side down, in the hot oil and brown 3-4 minutes, until potatoes are golden.
Gently flip fillets so the skin side is down and place pan in oven, until the centre of the fillet is opaque (approximately 10 minutes per inch or 2.5 cm at the thickest part of the salmon).
Serve immediately, potato side up.