Heat oven to 200C/180C fan/ gas 6. On a baking tray, sandwich a single layer of pancetta rashers between two pieces of parchment, then weigh it down with a second tray (this will stop it wrinkling as it cooks). Bake for 10-15 mins until the fat is crisp and golden. Pat with kitchen paper to absorb any excess fat. The pancetta can be cooked a day ahead and chilled. Reheat before serving.
Oil a 20cm round tin. Boil the stock in a saucepan, then slowly stir in the polenta and simmer for 5 mins, whisking, until thick and smooth. Stir in 1 tbsp oil, the rosemary and parmesan, if using, and season. Spoon the mixture into the tin, level the top, then set in the fridge for at least 1 hr in the fridge or until very firm.
When ready to eat, heat a griddle pan or non-stick frying pan until hot. Cut the set polenta into 16 wedges, brush with a little oil, then cook for 4 mins on each side or until golden and marked. Transfer to a serving plate or board. The polenta can be griddled a day ahead. Reheat it in a hot oven for a few minutes before serving.
Meanwhile, use a cutlery knife to push about 1 tsp cheese into the middle of each date half. Thread a cocktail stick through each stuffed date, then add a piece of pancetta, two spinach leaves and a wedge of hot polenta. To serve, drizzle with a little of the pomegranate molasses.