Simmer hamburger, onions, green pepper and oil until meat has lost red color. Sieve tomatoes and add with tomato paste and seasonings. Cover and simmer for 1 hour. Simmer livers and butter for 10 minutes and add to meat mixture. Cook spaghetti, drain and place in a 2 quart dish. Pour on meat sauce and toss lightly. Cover and bake 30 minutes at 350F.