In a large skillet over medium-high heat, heat 1/3 cup oil. Add chicken, season with salt and pepper, and sear until golden, 3 to 4 minutes, then flip and sear 3 minutes more.
Meanwhile, in a 6-quart slow-cooker, toss potatoes with remaining 2 tablespoons oil, butter, garlic, and thyme. Season generously with salt and pepper. Add chicken to slow cooker and toss gently, then cover and cook on high for 4 hours or low for 8 hours, until potatoes are tender and chicken is fully cooked.