Preheat the oven to 300 degrees F. Score the fish several times about 1/4-inch deep on both sides between the pectoral fins and tail to prevent it from curling while it cooks.
Season with the salt, pepper, lemon juice and a few tablespoons of olive oil, inside and out and place the fish in a baking dish.
In a saute pan, heat the remaining olive oil and sweat the onions and peppers until soft and translucent. Add the garlic, tomatoes and sweat until dry, stirring often so that it doesn't burn. Mix in the tomato paste and bay leaves and add about 4 cups of water. Let the mixture simmer and reduce by half.
Once reduced, spoon the mixture over the fish in the baking dish, as well as inside the cavity of the fish.
Sprinkle green onions, cilantro leaves, and lemon slices over the top. Place in the oven and bake for about 40 minutes.