Boil 1 rounded cup macaroni until tender. Brown 1/4 cup chopped green pepper, 1/2 cup chopped onion in 3 tablespoons oil. Add 2 tablespoons flour to 1 1/4 cups milk; stir constantly until thick. Add 1 can cream of chicken soup and stir until smooth. Add 1 can of drained tuna to cooked macaroni. Mix green pepper and onion to the mixture. Pour soup and milk over macaroni mixture; stir until blended and pour into greased casserole and bake at 350 degrees for 30 minutes.