Peel the potatoes and chop into 2cm slices. Put them into a pot of salted water and boil until the potatoes become soft.

Slice the onion, and fry in a pan with olive oil until browned.

To make the white sauce, put the margarine and flour into a pan on medium heat and stir until the mixture becomes sticky. Then add milk, season with salt, pepper and garlic powder. Continue to stir gently to avoid getting lumps in the mixture.

Preheat the oven to 200C/400F. Put the potatoes with the onions on a baking dish and pour on the white sauce. Finally add grated cheese on top. Bake for about 15 min or until the top has browned nicely.