Place half of beans in a bowl; mash with a fork or potato masher. Stir in remaining whole beans. Drain tomatoes, reserving 1/4 cup liquid.
Heat oil in a medium cast-iron or nonstick skillet over medium-high heat. Add onion and garlic; sauté 2 minutes or until tender. Add beans, tomatoes, 1/4 cup reserved tomato liquid, salt, and cumin; cook 3 minutes or until thoroughly heated, stirring constantly. Remove from heat, and let stand 2 minutes. Stir in sour cream.
Sprinkle cheese evenly over bean mixture; cover and let stand 3 minutes or until cheese melts. Sprinkle with cilantro, and serve warm with chips.