Heat oven to 425°F and set a rack in the middle. Put a rimmed baking sheet on the rack now (preheating the sheet helps catch drips without smoking).
In a large bowl, stir together the marinade ingredients: greek yogurt, garlic, lemon juice, cumin, smoked paprika, coriander, salt, pepper, cinnamon, and cloves. It should smell boldly spiced, not subtle.
Add ground lamb, grated-and-squeezed onion, olive oil, and baking powder. Mix until the meat looks evenly seasoned.
For true donair meat texture, you need a strong mix. Transfer to a food processor and pulse 12 times, scraping once, until it looks tacky and slightly smoother (it should start to clump). Alternative: beat in a stand mixer on medium for 3 minutes, or mix by hand for 5 minutes until sticky and bouncy. This mixing step is the difference between crumbly meatloaf and sliceable donair meat.
You can bake immediately, but if you have time, cover and refrigerate 20 minutes to let the spices bloom and the mixture firm up. This small rest makes shaping easier and edges crisper later.