Put the salt and about a gallon of water in a large container.
Stir until the salt dissolves.
Place the turkey in the brine, and refrigerate for at least 3 hours and up to overnight.
In a bowl, combine the oil, lemon zest, thyme, onion powder, garlic powder, cayenne, and black pepper.
Remove the turkey from the brine and pat dry with paper towels.
Rub the turkey with the oil and seasoning mixture.
Scatter the onion in the bottom of your crockpot.
Place the turkey skin side up and add the broth to the slow cooker.
Cover and cook on low heat for 6 hours.
Remove from the slow cooker and allow to rest for 20 minutes.
Preheat the broiler to high and broil for 3 to 5 minutes, until the skin is crispy.
Strain the liquid into a measuring cup. Add more broth as necessary to reach two cups.
Melt the butter in a saucepan over medium heat. Add the flour, and cook for 3 minutes, stirring often.
Add the broth, and whisk well. Cook over medium-low heat until thickened. Season to taste with salt and pepper.