Rinse the turkey with water, inside and out, then pat it dry with a paper towel. Place the bird breast-side down on a large board or flat work surface.
Next, cut both sides of the turkey's backbone with kitchen shears, starting at the end with the thigh, working your way up to the neck. Remove the backbone, and save for homemade stock or bone broth.
Grab both sides of the turkey, and open it like you would a book (butterfly). Turn the breast side up.
Push down on each side of the breast with your hands until you hear it crack. Then flatten the turkey.
If desired, remove the wishbone as it makes it easier to carve the whole breast in a single piece.
Rub inside and out of the turkey with salt. Set aside.
Combine the softened butter, garlic, sage, rosemary, thyme, parsley, paprika, salt, and pepper.
Let this mixture sit for a little bit (about 5 minutes). Generously rub the turkey inside and out with a little over half of the spice blend, then refrigerate until ready to cook.
Mix the onion, potatoes, carrots, and celery with the rest of the spice mix in a bowl. Set aside.
When ready to bake, roast the turkey on a baking rack; the turkey goes on top and the veggies below.
Roast at 400℉ (205℃) until the skin is crisp and an instant-read thermometer inserted into the thickest part of the breast reads 165℉ (74℃), 1-1½ hours.
Let it stand 5-10 minutes before slicing the turkey to serve.