Step 1

Season chicken on all sides with salt and pepper. Melt butter with oil in a large skillet over medium-high heat. Working in batches, add chicken and cook, turning often, until browned on all sides, 5 to 7 minutes. Transfer to a large slow cooker.

Step 2

Pour off all but 1 Tbsp. fat from skillet. Add onion and cook, stirring often, until softened, about 3 minutes. Add garlic and sauté for 30 seconds. Add tomatoes and bring to a boil, stirring to pick up any browned bits on bottom of skillet. Stir in beans and thyme.

Step 3

Pour contents of skillet over chicken in slow cooker. Cover and cook on low until chicken is tender and beans are soft, 3 to 4 hours. Transfer chicken to a platter and cover loosely with foil. Set cooker on high and bring bean mixture to a simmer. Cook, uncovered, until reduced and thickened, about 10 minutes. Season with salt and pepper. Discard thyme sprigs. Spoon beans over chicken on platter, then serve immediately.