Rinse salmon, pat dry, and lay skin down in a lightly oiled shallow 3-quart casserole (about 9 by 13 in.). Rub 1/4 teaspoon pepper over salmon.
Bake in a 400 oven until fish is opaque but still moist-looking in center of thickest part (cut to test), about 15 minutes.
Meanwhile, in a 1- to 1 1/2-quart pan over high heat, combine rosemary, marmalade, balsamic vinegar, and onion. Boil, stirring often, until reduced to 1/2 cup.
Cut salmon into equal pieces; use a wide spatula to transfer to plates. Spoon onion mixture over and around salmon.
