Preheat the broiler. Lay the salmon, skin-side down, on a baking sheet and season with salt and pepper. In a medium bowl, toss the onion with the dill and olive oil. Spread the onion mixture on the salmon and season with salt and pepper. Transfer to the lower third of the oven and broil for 5 minutes, rotating the sheet halfway through. Remove from the oven; leave the broiler on.