Heat the oil in a large heavy pot over moderate heat and saute the onions until lightly browned, about 10 minutes. Add the garlic and jalapenos and cook, stirring frequently, for 2 minutes.
Add the remaining ingredients and bring to a boil. Reduce the heat and simmer covered for 30-45 minutes. If you want a thicker stew, mash some of the beans against the side of the pot with the back of a spoon and stir them into the mixture.
Serve with any or all of the optional garnishes if desired.