Preheat the oven to 350 degrees F (175 degrees C). Grease a baking sheet, and set aside.
Heat oil in a skillet over medium-high heat. Saute onion with dried garlic, salt and pepper until tender. Transfer to a medium bowl, and mix with bread crumbs, mustard powder and mayonnaise. If necessary, add more mayonnaise to achieve a paste-like consistency.
Place salmon fillets onto the greased baking sheet, and press the crumb mixture on the top so that it is about 1/4 inch thick.
Bake for 10 minutes in the preheated oven, until salmon is easily cut with a fork, then broil for 5 minutes to crisp the top.