Melt butter over low heat, blend in flour, salt and pepper. Cook 1 minute, stir constantly. Remove from heat - gradually add milk, return to heat - cook until thick and bubbly.
Fold in ham, onion, green pepper and cheese. Pour over potatoes in large bowl. Stir gently - then pour in buttered 13 x 9 inch baking dish - cover with foil. Bake at 350 degrees for 30 minutes. Uncover and bake for 1 hour or until brown and bubbly.