Saute onions in a bit of olive oil until soft. Add garlic and saute another minute.
Reduce heat to medium-low and add beans, tomato with juice and spices.
Simmer on medium low until hot, stirring occasionally.

2 cans black beans, drained
1 can diced tomatoes
1 cup diced onion
2 cloves fresh garlic, minced
1/2 teaspoon cumin
1/2 teaspoon chili powder
salt & pepper to taste