Break tuna into pieces with a fork.
Arrange tuna, onion and pimentos over cooked green beans in a greased 2-quart casserole.
Cook bacon over medium heat until crisp and browned.
Remove bacon and add to tuna casserole; reserve drippings.
Add sugar and vinegar to bacon drippings.
Bring to a boil over medium heat and boil for 5 minutes.
Season with salt and pepper.
Pour vinegar dressing over ingredients in casserole.
Cover and bake in 350ºF oven for 30 minutes.