Ham, Potato and Broccoli Casserole

1 1/2 c. diced potatoes
1/2 c. slightly steamed broccoli
1 c. diced ham
1 T. chopped onion
1/3 c. butter
3 T. flour
1 1/2 c. milk
1 c. shredded cheese
3/4 tsp. salt
dash pepper

Layer 9x13 pan with ham, potatoes, and broccoli. In a saucepan, melt butter and saute onions. Stir in flour to form paste. Add milk all at once to avoid lumps and stir until thick and bubbly. Add cheese and salt and pepper and stir until cheese melts. Pour cheese sauce over casserole and bake at 350 degrees for 45 minutes or until potatoes are cooked.