Au Gratin Potatoes - 3.5 Points

Ingredients
1 tablespoon butter
1 medium onion(s), thinly sliced
2 level tablespoon flour
500 ml skimmed milk
1000 g potato(es), thinly sliced
125 g half-fat cheddar cheese, grated
1 teaspoon salt
1/4 teaspoon pepper

Instructions
1. Preheat oven to Gas Mark 5/190°C/375°F.
2. Melt butter in a large casserole dish over medium heat. Add onion and cook, stirring occasionally, until onion begins to brown, about 5 minutes. Stir in flour; add milk slowly, stirring. Add potatoes; stir to mix. Bring to the boil. Stir in ¾ of cheese, salt and pepper.
3. Level out surface. Transfer to oven and bake for 1 hour, uncovered. Cover and bake until potatoes are tender when tested with a fork, about 20 minutes more.
4. Preheat grill. Sprinkle remaining cheese over potatoes. Grill until the cheese is golden brown, about 1 to 2 minutes. Cool for 5 minutes before slicing into 8 pieces.