Mix together the lemon juice, sugar and soy sauce until the sugar has dissolved and marinate the salmon filets for 15-30 minutes.
Bring 3 cups of water to boil and stir in the rice, ginger, lemon grass, salt and butter and simmer covered for 15 minutes. Let rest covered for another 5-10 minutes. Fluff with a fork before serving
In a small sauce pan mix together the coconut milk, curry paste and fish sauce and simmer for 15 minutes to thicken slightly.
Heat a large, heavy skillet over high. Add 2 teaspoons of peanut oil and when it is shimmering hot, but not smoking, carefully drop the salmon filets into the pan. A grease guard is recommended. Allow the fish to caramelize for about 2 minutes. Turn the fish and spoon about 1/2 cup of the marinade over the salmon. If using a thick cut of salmon move it to a 350 degree oven for about 5 minutes to finish cooking.
Mix together the watermelon, green tomatoes, herbs, lime juice, salt and peanut oil.
To serve, remove the ginger slices and the lemon grass from the rice and plate the rice spooning the coconut curry sauce over it. Top with the salmon filets and the watermelon salsa on the side. A nice addition to the plate are a few blanched sugar snap peas.