Start by making the polenta. Heat the chicken stock in a medium saucepan until almost boiling and slowly add the polenta constantly whisking until combined. Reduce the heat to low, cover and cook for 15 minutes, whisking every 5 minutes.
Stir in the milk, butter and parmesan cheese and season to taste with salt. Spoon the polenta into a medium sized greased dish and place in the refrigerator for about 1 hour to set. Wait 20 minutes before starting the next steps.
Place the dried porcini mushrooms in a small bowl, cover with boiling water and set aside until mushrooms are plump and soft. Reserve the liquid for cooking.
Heat a splash of olive oil in a large pot over a medium-high heat and fry the onions until they are soft and translucent.
Add the minced beef and evenly brown the meat. Stir in the portobellini mushrooms, garlic, rosemary and chilli flakes and cook for a further 2 minutes.
Stir in the sundried tomatoes, beef stock, porcini mushrooms and porcini liquid and bring to the boil. Season with salt and freshly ground black pepper and gently simmer uncovered on a low heat for 20 to 30 minutes until the sauce has thickened.
In the meantime, pre-heat the oven to 230°C/450°F or turn on the grill.
Remove the polenta from the refrigerator and using a round metal cutter, cut the polenta into 9 circles.
Transfer the sundried tomato beef mixture into a medium oven-safe casserole dish, top with the polenta circles and sprinkle over the mozzarella cheese.
Place in the oven and grill for about 5 minutes or until the cheese is bubbling and golden. Garnish with herbs and serve whilst hot.