In a large pot, bring 5 1/4 cups of water to a boil over medium heat. Add the rice, turn the heat down to low and simmer until rice has absorbed all the water and is light and fluffy, about 15 to 20 minutes. Reserve 3 cups of cooked rice for another use, such as Cajun Dirty Rice. Transfer the rice to a serving bowl and set aside.
In a large skillet over medium-high heat, add 2 tablespoons of canola oil. Add the celery, green pepper, onion, garlic, and season with salt, pepper and cayenne pepper. Saute until slightly tender about 5 minutes. Add the fresh lemon juice and chopped parsley and cook for another minute. Reserve 1 cup of the sauteed vegetables for another use, like Cajun Dirty Rice. Transfer the vegetables to a serving bowl and set aside.
While the vegetables are sauteing, heat a cast iron skillet or griddle over high heat. Brush the catfish with the remaining canola oil and then sprinkle both sides with Cajun seasoning. Place the catfish onto into the hot pan and cook, about 2 to 3 minutes on each side.
Arrange the catfish on a serving platter and serve with rice and sauteed vegetables.