Heat the oil in a pan and sauté the onion, garlic and rice. Add the wine and reduce the heat. Add the stock or water a little at a time, stirring with a wooden spoon, until the rice has absorbed all the liquid. Season with salt and pepper.
Cut the prosciutto into strips and bake it in the oven with parchment paper at 200 °c for 5 minutes.
Remove from the heat and while the rice is still hot stir in the butter and half the feta. Divide the risotto among plates and top with the remaining cheese, the baked prosciutto and a little fresh thyme. Drizzle with olive oil.