Bring the turkey carcass and water to the boil in a large pot over high heat. Skim off and discard any scum that forms. Add the chopped celery, chopped carrots, chopped Spanish onion, 1/4 bunch parsley, bay leaves and peppercorns and return to a simmer. Reduce heat to medium-low and simmer uncovered for 3 hours. Strain the turkey stock through a mesh sieve and skim off any fat that floats to the surface.
Heat the olive oil in a large pot over medium heat. Stir in the red onion; cook until the onion has softened and turned translucent, about 5 minutes. Add the diced celery, diced carrots, parsnip and swede; cook 5 minutes more. Stir in the garlic and chopped parsley and cook for 1 minute more. Pour in the turkey stock, season to taste with salt and pepper and bring to a simmer over high heat.
Reduce heat to medium-low and simmer until the vegetables are nearly tender, 15 to 20 minutes. Stir in the orzo and cook until just tender, about 7 minutes. Cover and remove the pot from the heat. Let stand until the orzo is tender, about 5 minutes.
You can also use 1.5 kg turkey necks and wings instead of a turkey carcass.