Preheat An Oven To 350 Degrees F (175 Degrees C). Arrange The Catfish In A Shallow Baking Dish; Season Liberally With Salt And Pepper. Spread Onion Slices Over The Fish. Dot Fish With Butter. Bake In The Preheated Oven Until The Fish Flakes Easily With A Fork, About 20 Minutes. Remove Baking Dish From Oven. Preheat The Oven'S Broiler. Mix The Mayonnaise, Dijon Mustard, Lemon Juice, And Hot Chile Oil Together In A Bowl. Spread The Mayonnaise Mixture Over The Fish. Return The Fish To The Oven And Broil Until The Sauce Is Browned And Bubbly, 2 To 3 Minutes. Garnish With Chopped Cilantro.