In a small bowl, stir together softened butter, garlic powder, Italian seasoning, seasoning salt, and black pepper until smooth.
Pat the turkey dry with paper towels. Gently loosen the skin around the breast and legs (use the back of a spoon if needed). Rub half of the garlic butter under the skin and the rest over the skin. (If you can, do this step the night before! Cover and refrigerate overnight to let all that flavor soak in.)
Place the turkey breast-side up in your slow cooker. Pour ½ cup chicken broth or water into the bottom (this keeps it extra moist).
Cover and cook on LOW for 7 - 8 hours, or until the turkey reaches 170 degrees F in the breast and 180 degrees F in the thigh. (If you’re short on time, HIGH for 3½–4 hours works too!)
Transfer turkey to a baking sheet or roasting pan. Melt 1-2 tablespoons butter and brush over the skin.
Broil in the oven for 5-8 minutes, watching closely, until golden and crisp.
Strain the liquid from the slow cooker into a small saucepan.
Whisk in broth and cornstarch mixture, and simmer over medium heat for 3-4 minutes, or until thickened. Season to taste with salt, pepper, and a little poultry seasoning if desired.
Let the turkey rest for 15-20 minutes before slicing. Serve warm with homemade gravy.