Prep the oven and pan. Heat your oven to 425°F (220°C). Line a sheet pan with foil or parchment for easy cleanup.
Lightly oil the surface to prevent sticking.
Make the glaze. In a small bowl, whisk together brown sugar, soy sauce, Dijon mustard, minced garlic, lemon juice, olive oil, black pepper, and red pepper flakes if using. Stir until the sugar dissolves and the mixture is smooth.
Pat the salmon dry. Use paper towels to blot excess moisture. This helps the glaze cling and the fish sear nicely.
Season lightly. If using regular (not low-sodium) soy sauce, skip the salt or go very light.
Sprinkle just a pinch of salt and a little pepper on the salmon.
Glaze generously. Place the salmon skin-side down on the sheet pan. Spoon the glaze over the top and sides. Reserve a tablespoon or two of glaze for brushing at the end, if you like.
Roast until just done. Bake for 8–12 minutes, depending on thickness.
A good rule: about 4–6 minutes per half-inch of thickness. The salmon should flake easily but stay moist in the center.
Optional broil for caramelization. For extra color, switch to broil for 1–2 minutes at the end. Watch closely.
The sugar can go from perfect to burnt fast.
Finish and serve. Brush with the reserved glaze, if desired. Squeeze a little fresh lemon on top. Garnish with sliced green onions or chopped parsley.
Serve hot over rice, quinoa, or with a crisp salad.