Season the salmon with salt and pepper, going light on the salt. In a small bowl, combine the cornstarch with 1 tablespoon water until dissolved.
Heat the oil in a large skillet over medium high heat. Add the salmon and cook for about 5 minutes on each side, flipping it carefully, until cooked to your liking. Remove to a plate.
Return the pan to the medium heat. Add the garlic, lemongrass, ginger, and scallions and stir for one minute until you can smell the garlic. Add the soy sauce, 2 tablespoons water, brown sugar, mirin, and bring to a simmer. Stir in the cornstarch mixture. Reduce the sauce by one-third, about 2 minutes; it will thicken up nicely.
Remove from the heat and add the lime juice. Pour the hot sauce over the fish and serve with rice, if desired.
Notes
Leftover salmon can be kept in the fridge for 2 days. You can enjoy it at room temperature or warm it in the microwave for a minute. It can also be reheated in a preheated 300-degree oven for 5 minutes or so until warmed through. I don’t like to reheat cooked salmon until it’s super hot, as that tends to overcook the fish.