Wash and drain the pearl barley.
Finely chop the shallots, garlic and celery.
Heat the butter and oil in a heavy-based saucepan, then gently soften the chopped vegetables.
Add the pearl barley, paprika, thyme, bay leaf, chilli flakes and lemon zest. Stir for a minute, then then pour in the stock, the tinned tomatoes and passata.
Bring to the boil, then gently simmer over low temperature for about 45 minutes. Stir occasionally to prevent the barley pearls sticking to the bottom of the pan.
Meanwhile, dry-fry the cumin seeds in a pan, then crush them slightly in a mortar.
Cut the feta into 2-cm cubes, put them on a plate. Drizzle with the oil, then scatter the toasted cumin seeds and the oregano leaves over them. Set aside.
When the orzotto is soft & creamy, and it has absorbed almost all of the liquid, turn off the heat.
Taste it, and season with salt and pepper, if needed.
Serve warm with the marinated feta.