Preheat oven to 350 degrees F (175 degrees C). Grease an 8x 8 inch baking dish. Melt butter/oil in a medium saucepan. Remove from heat and mix in the flour and milk. Return to stove over medium heat, and stir until thickened. Season with pepper, garlic, and mustard.
Next add cheese to white sauce and cook on low until melted. (If desired save a bit of the cheese to put on top before baking.) Cut cod into bite sized pieces and gently stir into sauce.
Pour into baking dish and distribute fish evenly throughout. Bake in preheated oven for 20 to 25 minutes, or until top is beginning to brown.