Preheat the oven to 425 F. Mince the garlic and ginger well. Trim the tops of the lemongrass stalks and peel the outside of the bulb (lower part). Slice the lemongrass thinly and then continue to chop it into a mince. Slice the green parts of scallion and the cayenne chili (optional) and add it to a small bowl with the other ingredients. Add the kosher salt to the mixture and stir it well.
Step 2
Place the fish skin side up (if the skin is still attached) into a heavy casserole dish.
Step 3
Top the fish with the lemongrass mixture and bake it for 15-20 minutes or until the fish is roasted and golden brown on the edges.