Preheat oven to 375°F

Pour oil into casserole dish and thoroughly coat bottom and sides

In a saucepan melt butter and cook onions

In a small bowl add flour, salt, and pepper. Mix together.

Add flour mixture to the onions and stir until blended

Gradually stir in milk and cook stirring constantly until thickened

Layer 1/2 the potatoes in the casserole dish

Pour 1/2 sauce over the potatoes

Layer the remaining potatoes

Pour the remaining sauce over top

Cover and bake for 45 minutes

Uncover, add cheese and bake for another 15 minutes