Melt butter in a large skillet over medium-high heat. Add onion and sauté for 2-3 minutes, or until tender. Add garlic and cook for an additional minute.
Sprinkle flour on top and cook for 2 minutes then slowly stir in chicken broth. Season with thyme and bay leaves and stir to combine, then pour into slow cooker.
Season turkey breast with salt and pepper then place in the slow cooker, skin side up. Cover and cook on low for 5-7 hours or until turkey reaches an internal temperature of 165 degrees F.
After cooking, remove turkey from the slow cooker, place on a cutting board and cover with foil to keep warm. Let rest for 10 minutes.
While the turkey rests, make the gravy by straining the contents of the slow cooker into a saucepan over medium high heat. In a small bowl, whisk together cornstarch and water until combined then whisk into the saucepan. Bring to a boil, then reduce heat to low and let simmer until gravy reaches desired thickness.
Carve the turkey breast from the bone and serve with gravy.