Wrap one bacon slice around each chicken breast half and arrange in the crock of a slow cooker; add onion and garlic.
Stir mushroom soup, sour cream, and flour together in a bowl until smooth; pour into the slow cooker.
Cook on Low until chicken is no longer pink in the center and the juices run clear, 6 to 8 hours. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C).
Remove bacon-wrapped chicken to a serving platter. Beat sauce in slow cooker with a whisk; pour over chicken.