Heat the oil in a large pot, add the peppers and onions, and lower the heat to medium-high. Sauté for 2 minutes stirring frequently. Add the curry paste/powder, tomatoes and garlic and sauté for an additional minute. Add the coconut milk, chicken stock, lemongrass, ginger, kaffir lime leaves and salt to taste. Stir gently. Bring the soup to a boil, then set the heat on low and simmer for 15 minutes stirring from time to time.
If using chicken, toss the chicken into the soup and simmer until the chicken is cooked through. If using fish, add the fish and cook for 3-4 minutes or until the fish is soft and tender (be careful to not overcook; fish can easily become tough and chewy if cooked for a long time).
Remove the lemongrass and kaffir lime leaves from the soup, add the lime juice and serve with fresh cilantro leaves, lime wedges and steamed rice.