Heat a saucepan over medium heat, and add the bacon pieces. Fry for a few minutes, then add the onions. Cook and stir until the onions are tender, about 5 minutes. Pour in the green beans, and crumble in the bouillon cube. Stir to blend, then simmer for about 15 minutes. If you wish to simmer longer, add more water to keep the beans from scorching.
