Line a 20cm/8in square tin with baking parchment or heatproof cling film. Bring the stock to the boil in a large saucepan. Reduce the heat to a fast simmer and, using a whisk or silicone spatula, gradually stir in the polenta. Add the rosemary and season with salt and pepper.
Cook for 4-5 minutes, stirring constantly until large volcanic-like bubbles form. The polenta will swell and become really thick - don’t leave for a minute or it could become lumpy. Pour the mixture into the prepared tin and smooth the surface with the back of a spoon. Set aside to cool for 20 minutes.
Meanwhile, make the gremolata. Place the garlic, parsley and lemon zest in a pestle and mortar and pound the mixture to a mash so that all the flavours mix together. Stir in the oil. Alternatively, blend everything together in a mini-blender until as fine as possible. Either way, spoon into a small serving bowl and set aside.
Line a large baking tray with baking parchment. Once the polenta is firm, pull it up out of the tin, place on a chopping board and carefully remove the paper or cling film. Cut the slab into 10x2cm/4x¾in wide strips and then cut them in half across to give 20 chunky chips. Being careful not to break the chips, wrap a sage leaf around the middle of each one, followed by a slice of pancetta. Arrange on the tray as you go. These can be prepared to this stage a day ahead of time, in which case, cover with cling film and keep in the fridge.
Preheat the oven to 110C/225F/Gas ¼ (fan 80C). Heat two large frying pans on a medium to high heat and drizzle a tablespoon of oil into each one. Fry 10 polenta chips in each pan for about 1-2 minutes on each side. Use a small fish slice to carefully turn them over to avoid breaking them. Arrange them back on the baking tray once cooked and pop them in the oven to keep warm.
For the fish and asparagus, wipe out the frying pans with kitchen paper and return both to a medium to high heat with a tablespoon of oil in each one. Season the salmon with salt and pepper and, once the oil is hot, place the fish skin-side down in one pan and leave to cook for about 3-4 minutes.
Meanwhile, for the asparagus, fry the asparagus in the other pan for 3-4 minutes tossing occasionally.
If you want to make the sauce, simply melt the butter in a small saucepan and add the finely chopped parsley, lemon juice and season to taste. Keep warm.
Once the fish has had its time, flip it over and let it cook for four minutes on the other side.
Meanwhile, remove the polenta chips from the oven and arrange five on each serving plate. Divide the asparagus up evenly between them also. Once the salmon is cooked to your liking (either a little pink in the centre or cooked completely through) remove from the heat and arrange one on each serving plate. If you made the optional sauce, pour this over now.
Serve at once with the gremolata and lemon wedges.