Place onions and vinegar in a medium bowl and stir to combine. Let sit for 15 minutes, while you prep the rest of the ingredients.
Add the beans, bell peppers, capers, mint and olive oil to the bowl with the onions. Sprinkle with salt and pepper and stir to combine. Taste and adjust the seasonings.
Notes
Beans - can use great northern or cannellini.Mint - can substitute basil or parsley.Red bell pepper - fresh or jarred roasted. Can had a can of tuna and turn it into a main dish salad. Variations - feta, goat cheese, kalamata olives.