1 tablespoon vegetable oil
1 cup diced yellow onion
2 cloves garlic, minced
2 (15-ounce) cans pinto beans, rinsed and drained
2 (15-ounce) cans red beans, rinsed and drained
2 (15-ounce) cans cannellini beans, rinsed and drained
2 (12-ounce) bottles root beer (not diet)
½ cup apple cider vinegar
½ cup ketchup
3 tablespoons firmly packed light brown sugar
2 tablespoons Dijon mustard
2 teaspoons chili powder
1½ teaspoons kosher salt
1 teaspoon ground black pepper
Preheat oven to 400°.
In a large Dutch oven, heat oil over medium-high heat. Add onion; cook, stirring frequently, until lightly browned, about 8 minutes. Stir in garlic; cook for 1 minute. Stir in beans and all remaining ingredients; bring to a boil. Transfer pot to oven.
Bake, uncovered, until liquid is thickened, about 45 minutes. Stir before serving.