Flip the salmon and cook for 2 minutes. Add the garlic and cook until fragrant but not browned, about 15 seconds. Pour the sauce over the salmon. Cook, spooning some of the sauce over salmon as it cooks, until the sauce is thickened and reduced by about half, and the salmon is just cooked through, 3 to 4 minutes. An instant-read thermometer into the center of the thickest fillet should register 120°F to 130°F for medium-rare, or 135°F to 145°F for more well-done. Garnish with thinly sliced scallions if desired.
