In a large pot, melt butter over medium heat. Add onion, bell pepper, and celery. Cook for 30 minutes, stirring frequently, until vegetables are very tender and slightly brown. Mix in mushrooms, dices tomatoes, garlic, salt, black pepper, and cook for 10 minutes. Remove from heat and mix in cooked rice to desired consistency.
Meanwhile, in a large skillet over medium heat, saute catfish for 3-4 minutes per side or until fully cooked. Fold cooked catfish into rice so as not to break the catfish completely. Our Catfish Jambalaya should serve 6 to 8 people.