Preheat the oven to 475 degrees F. Heat a large cast-iron skillet over high heat. Rub the turkey skin with 1 tablespoon olive oil; rub over and under the skin with the marjoram, garlic, salt and pepper.
Sear the turkey, skin-side down, without moving, until golden, 4 to 5 minutes. Flip the turkey, then add the onions, cut-side down, and cook until slightly browned, 4 to 5 more minutes. Scatter the celery and carrots over the onions, drizzle with 1 tablespoon olive oil and season with salt and pepper. Add the mushrooms, drizzle with the remaining 1 tablespoon oil and season with salt and pepper.
Transfer the skillet to the oven and roast until a thermometer inserted into the thickest part of the turkey registers 160 degrees F and the vegetables are tender, 20 to 25 minutes. Let the turkey rest 5 minutes before slicing. Sprinkle with the parsley.