Cook bacon in frying pan until crisp. Drain bacon and set aside, leaving drippings in skillet. Add onions to drippings and saute until tender. Add green beans to pan and saute over medium heat for 2 minutes, stirring frequently. Add boiling water to pan and cover. Cook for about 15 minutes, just until beans are tender. Add vinegar, butter, and salt and cayenne, to taste, and stir. Add back crisp bacon before serving. Garnish with a squeeze of lemon juice.