Make sure the turkey breast is completely thawed -- this can take 1-2 days in the fridge so be prepared. It isn't food safe to thaw a turkey in the crockpot. See notes in the post for more details about thawing your turkey breast. Pat the turkey down with paper towels.
Spray a large crockpot (I use a 6 quart crockpot) with nonstick spray.
Place the celery at the bottom. Cut one yellow onion into large chunks and add it to the bottom. Add the baby carrots. Pour the chicken broth on top.
Cut off the skin of the turkey breast (OR leave it on, whatever you prefer -- see last paragraph of blog post for more notes on this)
Place the turkey breast, BREAST DOWN on top of the vegetables. Make sure the turkey is ELEVATED on top of the onion/vegetables.
Cut the other onion in half and place inside the turkey with 4 tablespoons butter.
Stir together all of the seasonings together. Add in the melted butter and stir. Rub all over the turkey. If leaving the skin on, make sure to rub the seasonings under the skin too.
Cover the crockpot and cook on high for 1 hour and then low for 5-7 hours or until the turkey's internal temperature reaches 165 degrees F.
Remove the turkey from the crockpot and turn so it is breast up.
Start by removing the wishbone and then remove all the other bones. Take the large pieces of meat and slice them.
TO MAKE GRAVY: Strain everything from the crockpot out into a large bowl (press down the veggies to extract all the flavor/liquid). Measure that liquid (you should have right around 2 cups) and then set aside. In a large saucepan, add 4 tablespoons butter and melt. Whisking constantly, add in 4 tablespoons white flour. Gradually add in the strained liquid from the crockpot, whisking constantly. Add salt, pepper, and any other seasonings needed. Serve warm with the turkey.