Preheat a gas grill to high, build a fire in a charcoal grill or build a campfire and let it burn down to high heat (about 500 degrees F).
Brush or drizzle salmon with oil and sprinkle with 1/4 teaspoon salt. Sprinkle peppers and onions with the remaining 1/4 teaspoon salt.
Oil the grill rack (see Tip). Place the salmon, skin-side up, on the grill, along with the peppers and onions. Grill the salmon until browned, 3 to 4 minutes. Using a metal spatula, gently nudge one of the pieces: it should release from the grill without much force, but if it feels stuck, continue cooking for another minute. When the salmon releases easily, flip and continue cooking until browned and the flesh is opaque, about 3 minutes more. Cook the peppers and onions, turning frequently, until tender and well browned, 8 to 10 minutes total.
Serve the salmon with the vegetables.
Tips in page 4