Use paper towels to thoroughly pat dry the thawed turkey breast. Combine 2 tsp salt, 1 tsp black pepper, and 1 tsp poultry seasoning in a small bowl. If your turkey breast has an internal cavity, sprinkle the spice rub generously on the inside of the turkey breast, leaving more to sprinkle on the outside later. If your turkey breast does not have an internal cavity, move on to step 2.
Combine the herb butter ingredients in a small bowl. Loosen skin from the breast meat of the turkey by running a small spatula gently underneath the skin. Spread about half of the herb butter underneath the skin. Use your fingers to smush it around on top of the skin.
Rub the remaining herb butter all over the outside of the turkey and on top of the skin. Generously sprinkle more of the seasoning mix over the buttered turkey.
Place the onion wedges in the bottom of the crockpot, and then place the prepared turkey on top.
Cover the crockpot and cook on low heat until a thermometer inserted into the breast meat reaches 160-165 degrees, about 4-6 hours. You can also cook on high heat for 3-4 hours, just be sure to keep an eye on the internal temperature of the turkey.
Preheat your oven broiler and place the top rack close to the heat source leaving enough room for the turkey to fit underneath. Remove the turkey to a roasting pan and broil for 3-6 minutes, watching closely to make sure it doesn’t burn. You just want a nice golden brown color on the skin. Remove from the oven and allow the turkey to rest for 10-15 minutes before slicing and serving.
To make the gravy, strain the drippings (excluding the onions) from the crockpot into a liquid measuring cup using a mesh strainer. If you do not have 2 cups of drippings, then add enough chicken stock into the measuring cup to make 2 cups of liquid. I had right at 2 cups of drippings from my turkey and did not need any extra chicken stock.
In a small saucepan, melt ¼ cup (½ stick) of unsalted butter over medium low heat. Whisk in ¼ cup all-purpose flour and cook for 1 minute.
Slowly pour the turkey drippings while whisking constantly to avoid lumps. Cook until simmering and thickened. Taste, and add salt and pepper to taste if necessary. Serve immediately with the sliced turkey.