Set a pot on medium heat and pour in vegetable oil. Saute bell pepper and onion slices to gain a little color, about 3 minutes Add garlic and let cook briefly, maybe ten seconds, then dump in beans with one can's liquid.
Add cumin and stir, then raise heat to medium high. Allow to come to a simmer, then take the heat to low and cook for thirty minutes. Don't let the liquid run out (it shouldn't be hot enough to boil)
After thirty minutes kitchen should be very fragrant, add salt and pepper to taste and the lime juice.