In a food processor or blender, combine 2 cups (480 ml) of the beans with about 1/2 cup (480 ml) of the bean-cooking liquid or vegetable stock.
Blend until smooth, adding more liquid as needed to reach a puréed consistency.
In a separate medium bowl, lightly mash the remaining 1 cup (240 ml) of beans with a fork or potato masher to create some texture.
Scrape the puréed beans from the food processor into the bowl with the mashed beans.
Add the minced red bell pepper, onion, optional tomato, fresh chile, and vinegar.
Stir everything together until well combined.
Season generously with salt and freshly ground black pepper.
Taste and adjust the seasoning as needed. If the dip is too thick for your liking, stir in a little more bean liquid or stock until you reach the perfect consistency.
Serve immediately with your favorite dippers, or cover and refrigerate for up to 3 days.
Bring to room temperature or reheat gently before serving.
In a hurry? Swap the minced pepper, onion, tomato, chile, and vinegar for 1 ½ cups of your favorite fresh salsa for a super-fast version.