Heat oil in the pot. Add flour and stir with a wooden spoon until the mixture becomes mushy. If it's too dry, add more oil.
Once the mixture is mushy, add milk and crushed bouillon (if you use bouillon in your kitchen, if not, use salt, pepper, and vegeta). Cook on the medium-low.
We usually mix the sauce with a stick blender to get rid of the lumps. But you can also leave the sauce this way or just use a potato masher.
When the sauce starts to bubble, add horseradish. Cook for 2 more minutes and stir well.
