Pat the pork roast dry with a paper towel and pierce the meat at 2 cm intervals with a knife or fork. Insert leaves of rosemary in the perforations.
Place the onion halves into a glass 23 or 25 cm pie dish. Set the meat on top of the onions so they act as a roasting rack. Pour the water over the meat and season with salt, pepper, thyme and cumin.
Cook in the microwave at full power for 16 minutes (or about 8 minutes per 500g). Turn the roast over and cook for an additional 6 minutes. Let the roast rest for about 15 minutes before carving and serving.