Preheat the oven to 180°C (356°F) top and bottom (fan oven not recommended). Sprinkle half the thyme leaves over the veal loin steaks.
Heat the sunflower oil in a roaster and brown the vegetables with the tomato purée over a high heat for approx. 3 minutes. Cover with the veal stock, add the veal and cook everything in the oven for approx. 35 minutes (core temperature should be approx. 58°C (136°F) for medium and 65-70°C (149-158 °F) for well done). Repeatedly drizzle the meat with some stock from the roaster.
Sweat the pumpkin and garlic in hot olive oil over a medium heat for approx. 2 minutes. Season with salt and pepper and cover with the lid. Braise over a low heat for approx. 15 minutes, stirring occasionally.
Remove the veal from the roaster and wrap in foil. Sieve the veal stock into a pan, remove the lid and reduce the sauce over a high heat for approx. 5 minutes, then thicken with the ice-cold butter. Arrange the meat and vegetables in sauce on plates and serve with dumplings, for example.