Ask your butcher to cut the pork neck lengthwise into thin slices resembling roulades, keeping them stacked together.
Mix the gyros spice with the olive oil in a bowl, then season with salt and pepper to taste to create a smooth marinade. Refrigerate the marinade until ready to use.
Layer the pork slices: place the first slice down, brush only the visible side with marinade, add a layer of onions and red cabbage, then repeat with the next pork slice until all ingredients are used.
Tie the roast with kitchen twine: place knots at the beginning, end, and middle of the roast. Keep these 3 knots loose to prevent the filling from pushing out. Then bind along the remaining length of the roast at regular intervals with firm tension.
Preheat the oven to 180°C (356°F) with top and bottom heat. Place the roast in the preheated oven and roast for 1.5 to 2 hours until the thickest part reaches 63°C (145°F) internal temperature. Baste with pan juices every 15-20 minutes. The roast will shrink as the onions release water.
Remove the roast from the oven and let rest for 5 minutes. Working from one twine section to the next, slice the meat into small pieces.
Serve with rice or flatbread, salad, and tzatziki.