1. Preheat the oven to 425°F, making sure to place a rack in its middle. In an 8×8-inch baking dish, arrange the frozen salmon fillets with the skin side facing down.
2. In a small mixing bowl, combine the mustard, garlic, maple syrup, red pepper flakes (if you choose to use it), and salt.
3. Spread the resulting mixture evenly on the salmon using a brush or a spoon.
4. Aluminum foil should be tightly wrapped around the baking dish. Roast the salmon filets for 15 minutes or until they can be pierced through entirely with only a small, frosty resistance.
5. Remove the foil with care. Continue roasting the salmon for another 8 to 10 minutes, or until the salmon filet can easily be split with a fork or until a probe thermometer placed in the thickest portion of the fish reads 145°F. Because wild salmon cooks faster than farm-raised fish, start monitoring it at 8 minutes. The flesh should ideally be moist but not transparent or raw.
6. Take the salmon out of the oven and set it aside to rest for 3 minutes, uncovered. Serve alongside a dish of quick pasta or a hearty salad.

If you’d like some more inspiration on working with frozen salmon filet in your recipes, you may be interested in this video recipe.