Cook sausage and onion in a large saucepan over medium heat, covered, stirring occasionally, until fat renders, 8 to 10 minutes. Uncover and continue cooking, stirring occasionally, until browned, 5 to 7 minutes more. (If bottom of pan starts to burn in spots, sprinkle with water and scrape up blackened bits.)
Add collard greens, beans, and 4 cups water; season with salt and pepper. Bring to a boil, reduce heat, and simmer until soup is slightly thickened, 5 to 7 minutes. Adjust consistency of soup with water, if necessary. Stir in red-wine vinegar, and serve with toasted bread slices, if desired.
If collards are not available in your area, substitute other frozen chopped greens such as kale or turnip or mustard greens.