Wash the meat, dab dry and freeze in cling film for about 2 hours.
Mix gyros spice, salt, pepper and oil. Peel onions and slice or cut them into thin rings. Cut meat into very thin slices. In a large bowl, mix the seasoning oil and onions. Chill for about 1 hour.
Layer the meat and half the onions alternately in a large casserole dish, pressing well. Spread the rest of the onions on the meat. Cover with aluminium foil.
Braise in a hot oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer) for approx. 1 hour. Remove foil, continue to fry for approx. 30 minutes. Drizzle with gravy more often. Leave the roast to rest for a while, then cut into slices (the slices will fall apart slightly).