Heat the oil in a large skillet over medium-high heat until it shimmers. Add the onion, celery, and garlic and cook, stirring, until softened, 2 to 3 minutes. Add the vegetable broth and tomato paste, stir until well blended, and bring to a boil.

Add the catfish to the skillet and reduce the heat to medium. Add the lemon juice and at least a few red pepper flakes (use the full teaspoon if you like things spicy). Cover and simmer until the catfish flakes with a fork, 10 minutes or more depending on thickness.

Put the hot rice on a platter. With a spatula, remove the fish from the skillet and put it on top of the rice. Raise the heat under the skillet to medium-high and bring the remaining liquid to a boil, scraping the bottom of the pan and stirring until the sauce is well blended. Ladle the sauce over the fish and rice and garnish with the parsley.