Preheat oven to 400°F. Line a 9×13-inch baking dish with a sheet of parchment paper (or foil) for easy clean up.
Remove the salmon from the refrigerator and let it come to room temperature for 20 minutes.
4 salmon fillets
Meanwhile, in a small saucepan, combine the maple syrup, butter, soy sauce, Dijon mustard, garlic and red pepper flakes. Heat over medium heat, whisking often, until bubbly, then continue cooking, whisking constantly, for 1-2 minutes, until slightly thickened. (The sauce will thicken up a bit more as it rests.)
¼ cup pure maple syrup, 3 tablespoons unsalted butter, 2 tablespoons low sodium soy sauce, 1 tablespoon Dijon mustard, 3 cloves garlic, ¼ teaspoon red pepper flakes
Remove pan from the heat and whisk in the lemon juice. Reserve about 3 tablespoons of the glaze in a small bowl for drizzling when the salmon comes out of the oven.
2 teaspoons fresh lemon juice
Place salmon, skin side down, in the prepared baking dish. Pat dry with paper towels. Brush a thin layer of the glaze over the tops of the salmon fillets.
Bake salmon for 10 minutes. Then brush with more of the glaze from the saucepan. Continue baking for 2-8 more minutes, or until salmon is cooked through to 145°F, brushing with more glaze every few minutes. The cook time will depend on the thickness of the salmon fillets. Discard any extra glaze left in the saucepan.
Serve salmon with the glaze that you reserved in the small bowl. Enjoy!