Deep-fry the whole snapper until lightly brown, approxmately 4 minutes.
Saute the onion, pepper, thyme, 1/2 c water and 1/4 c of oil in a large saute pan, for about 2 minutes. Add the fish sauce and the fried fish. Simmer for 5 minutes, then add the tomato, and let simmer for 1 minute. Season with salt and pepper , to taste, and place on a serving platter.