Wash the meat, dab dry and wrap in foil. Freeze for about 2 hours. Mix gyros spice, 2 level teaspoons of salt, 1 teaspoon of pepper and oil. Peel onions and slice or cut into thin rings. Cut the frozen meat with a slicer or a large sharp knife into thin slices of approx. 1⁄2 cm.
Mix meat slices in a large bowl with spiced oil and onions. Cover and chill for about 1 hour.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Layer the meat and some onions in a large casserole dish or on a fat pan (deep baking tray). Spread the remaining onions on the meat. Cover with foil.
Cook in the hot oven for about 1 hour. Then remove the foil and continue cooking for approx. 30 minutes, sprinkling the gravy several times. Remove the roast and let it rest for about 5 minutes. Then cut into thin slices. Goes well with flat bread and shepherd's salad with tzatziki sauce - or you can buy ready-made tzatziki and coleslaw.