1. Reline baking sheet and coat with oil spray.
2. Working with one piece of salmon at a time, place fish on a clean surface, sprinkle lightly with salt and garlic powder. Lay two of each color pepper strips crosswise in center.
3. Roll fillets jelly-roll style and place seam-side down on prepared baking tray. Smear top of fish roll with mustard and sprinkle generously with minced onion and garlic. Repeat with remaining slices.
4. Lower oven temperature to 375°F (190°C) and bake fish uncovered for 12–15 minutes. Serve hot or warm.