Heat 1 ½ tablespoons of the oil in a large skillet. Add the onion and sauté over medium-low heat until it is translucent. Add the bell pepper and garlic and continue to sauté until the onion is golden.
Add the beans, vinegar, and seasonings (be generous with the black pepper) along with the water or cooking liquid. Simmer over very low heat, covered, for 10 minutes.
With the back of a wooden spoon or a mashing implement, mash about ½ cup of the beans in the skillet, or enough to thicken the base. Make sure there is enough liquid to keep everything nice and moist but not soupy. Simmer for 5 minutes longer.
Serve the black beans over the cooked rice, topping each serving with a sprinkling of the chopped raw onion, if desired.