Step 1

Heat oil in a large, heavy pot over medium heat until hot; add onion and bell pepper. Season generously with salt (about 1 teaspoon). Cook slowly, stirring often, until vegetables are a light golden brown (about 15 minutes). Stir in garlic, cumin, oregano, vinegar, beans, and 8 cups (2 quarts) water. Season very generously with salt (about 2 teaspoons). Cover and bring to a boil. Reduce heat to slowly simmer, uncovered and stirring occasionally, until beans are just cooked through but firm (about 1 hour).

Step 2

Raise heat to quickly simmer, uncovered and stirring occasionally, until beans just begin to break apart and broth has reduced to a thick, flavorful gravy (about 1 hour). Season soup with additional salt and vinegar to taste.