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.
Coat the salmon with the mustard seeds, patting the seeds to help them adhere. Return the salmon to the oven and broil, rotating the baking sheet, for about 5 minutes, or until the fillet is just cooked through. Serve the salmon at once with the Lentil Salad.
The richness of the salmon and the earthiness of the mustard seeds and lentils are best matched by a full-bodied, crisp white, such as a Sauvignon Blanc.