Preheat your oven to 425⁰F (220⁰C). If you have a fan setting, use it. It will help the fish heat and cook more evenly.
Next, line a baking dish with non-stick baking paper, a silicone mat, or a layer of aluminum foil. This makes clean-up A LOT easier. Set it aside.
Make the basting sauce by combining the mustard, honey, minced garlic, pepper flakes, salt, and pepper. Whisk them together well so the flavors are evenly distributed.
Place the frozen salmon fillets into the baking dish, skin-side down. Then, brush the top of the fillet (flesh side) with your homemade sweet and tangy basting sauce. Make sure you add it evenly and that it covers the entire surface.
Cover the baking dish with foil. Place the fillets inside the oven and leave them to bake (steam) for 25-30 minutes. To test the fish's doneness at this point, a sharp tip of a knife should easily pierce the flesh. It shouldn't feel any resistance from ice crystals.
Next, remove the foil and place the tray back in the oven. Leave the fillets to roast for 8-10 minutes, possibly longer.
Test the internal temperature of the salmon. Wild salmon should be at least 120⁰F (49⁰C) and 125⁰F (52⁰C) for farmed salmon.
Remove the beautifully cooked fillets from the oven and let the salmon rest for 5 minutes.
Serve the fish immediately with your favorite accompaniment and drink.