Preheat oven to 425ºF and set a rack in the middle. Add the sliced leeks, fennel, garlic cloves and lemon slices to a 9 x 13-inch glass, ceramic or metal baking dish and drizzle the oil over the vegetables. Sprinkle with salt and pepper to taste, and roast for 10 to 12 minutes, until the vegetables soften and char slightly around the edges. Remove the dish from the oven and lower the temperature to 225ºF.

While the vegetables are roasting, make the glaze. In a small bowl, stir together the pomegranate molasses, mustard, sugar, and vinegar. Season the salmon generously with salt and pepper. Lay the salmon on top of the fennel and brush the glaze over it. Drizzle a little more olive oil over the fish, if desired. Roast until the salmon is just cooked through and barely opaque in the center (about 125ºF on an instant-read thermometer). Cooking times will vary from 15 to 20 minutes for small fillets, or up to 40 to 50 minutes for a large piece. The fish is done when the edges look opaque and the thick part of the salmon flakes easily (125ºF on an instant read thermometer). If you want a darker glaze, you can pop the pan under the broiler for a few seconds. (Watch it carefully so it doesn’t burn.)

Serve the fish with the leeks, fennel and lemon on the side or underneath, with some reserved fennel fronds as garnish.