Fresh rosemary, thyme, and oregano give salmon fillets a savory flavor boost.
Make three to four 2-inch-long, 1/4-inch-deep, evenly-spaced slits along the top of each salmon fillet.
In a shallow dish, whisk together mustard, lemon juice, thyme, rosemary, oregano, salt and pepper. Add salmon and turn to coat both sides. Cover with plastic wrap and refrigerate 15 minutes. (If you are using a whole fish, do the same. The marinade will seep into the slits.) Reserve marinade.
Preheat oven to 450° F. Coat a shallow baking pan with cooking spray. Arrange onion and tomato slices in the bottom of the prepared pan. Place salmon on top of onion and tomato. Pour remaining marinade over salmon. Roast 10 to 15 minutes, until fish is fork-tender.