Prepare the Crock Pot: Place the sliced onion and whole garlic cloves in the bottom of the slow cooker.
Prepare the Turkey: Rinse the turkey breast and pat it dry with paper towels.
Salt and pepper the entire breast and rub it with olive oil.
Carefully lift the skin of the turkey breast and place slices of butter underneath.
In a small bowl, combine garlic powder, onion powder, paprika, and thyme. Rub this mixture all over the turkey breast.
Cook the Turkey: Place the seasoned turkey breast on top of the onions and garlic in the slow cooker.
Cover and cook on high for 3 hours, or until the internal temperature reaches 165°F (use an instant-read meat thermometer to check). It's critical you don't overcook the breast otherwise it will be dry and difficult to eat.
Optional: After cooking, you can place the turkey breast under the broiler for a couple of minutes to crisp up the skin.
Prepare the Gravy: Once the turkey is cooked, carefully remove it from the slow cooker and transfer it to a serving platter.
Strain the onions and garlic from the liquid in the slow cooker to use as the base for the gravy. Make sure you have at least 2 cups of liquid; if not, add water or chicken broth to make up the difference.
In a small saucepan, melt the butter over medium heat. Whisk in the flour and let it bubble for about a minute.
Gradually whisk in the turkey drippings (or chicken stock/water) until the mixture begins to boil and thickens into a gravy.
Taste the gravy and season with salt and black pepper as needed.
Serve: Slice the turkey breast against the grain and arrange the slices on the serving platter.
Drizzle the gravy over the sliced turkey for an excellent presentation.
Serve with your favorite Thanksgiving sides.