Cook 3 lbs. Potatoes. Cool and peel and dice.
Make cheese sauce by melting in saucepan ¼ 1/4 c. butter. Add and cook until thickened 5 T. flour. Gradually add, stirring constantly 3 c. milk. Add and cook on low heat until thick and smooth 1 t. salt and 1/8 t. pepper.
Add and allow to melt 1 c. grated cheese.
Arrange alternate layers of potatoes, cheese sauce and ½ 1/2 c. onion diced, in greased casserole dish.
Sprinkle top with ½ 1/2 c. grated cheese.
Bake at 375° for 25 minutes