Place salmon, skin side down, in a buttered baking dish, evenly sprinkle on the salt. In small bowl, add mustard, pepper, allspice, and garlic then mix until combined. Evenly spread over top of salmon, place in refrigerator to marinate for 30 minutes.

Using bake setting, preheat oven to 400 degrees F.

While salmon marinates, in a medium skillet and on medium heat, toast walnuts until just browned and fragrant. Set aside on a plate. Remove pan from heat, let cool for 1 to 2 minutes then wipe with paper towel to remove any nut residual.

Adjust temp to medium-low, add butter to same pan. When butter begins to foam, add sage leaves. Cook for about 5 to 7 minutes until foam subsides and butter starts turning brown. Remove from heat. With slotted spoon to let butter drain out, transfer crisped sage leaves to plate containing walnuts.

Add maple syrup to the browned butter and mix. Place and separate the apple slices in the pan, toss with sauce to coat, add pinch of salt to taste. Set aside in pan, allowing residual heat to slightly soften the apple slices. Let cool for 5 minutes.

Remove salmon from refrigerator. Layer the apple slices over the salmon, covering entirely. On middle rack, cook salmon for 15 to 20 minutes until internal temperature reaches 130 degrees F. Note: Cook time varies by thickness; figure about 8 to 10 minutes per inch of fish.