Have the garlic heads crosswise. Peel and halve the onions. Place garlic, onions, and thyme in the bottom of a 7-qt slow cooker. Add the broth.

Pat the turkey dry and season with salt (in moderation) and pepper. Place turkey on top of the onions and drizzle the honey all over. Cover and cook on low 6 hours or on high 3 hours.

Preheat the broiler to high. Remove the turkey breast and place it on a foil-lined baking sheet. Broil 2-3 minutes, until the skin is browned. Let stand 10 minutes before slicing.

Meanwhile, remove the onions, garlic, and thyme from the cooking liquid in the slow cooker. Whisk in the cornstarch and mustard together, then whisk into the cooking liquid. Cover and cook on high for 10 minutes until slightly thickened and coats the back of a spoon. Serve gravy with turkey.