about 500g, peeled & coarsely grated
Preheat the oven to 200°C/gas mark 6. Grease a 12-cup muffin tin liberally and line a baking tray with non-stick baking paper.
Heat the oil in a large sauté pan set over a medium heat. Add the celery, carrot, onion and garlic and fry for 6 to 8 minutes, until the veg are starting to soften.
Remove to a bowl, then add the beef mince to the pan, breaking it up with a wooden spoon. Fry until brown, then add the vegetables back to the pan along with the stock, tomato purée and Worcestershire sauce.
Simmer for 15 to 20 minutes, until the liquid has been soaked up. While the mince is cooking, mix the sweet potatoes and eggs together in a medium-sized bowl, then season with salt and pepper.
Add 3 tablespoons of this mixture to each muffin cup. Shape using your fingers or a teaspoon to make a nest in the middle, bringing the sweet potatoes up the sides of each cup.
Bake in the oven for 15 minutes, then remove from the tin and place on the lined tray. Divide the mince between the cups and sprinkle with the cheese.
Return to the oven for a further 8 to 10 minutes, until the cheese is golden and melted. Serve warm.