Start by making sure your fish fillets are ready. If fish comes from frozen, take out of the freezer and thaw overnight in the fridge or placed in cold water for about 20 mins- 1 hour before cooking it. Since this it will be fried it has to be fairly dry so pat dry with kitchen towel and then salt lightly and set aside.
Let's prepare the garlic sauce now which has to be ready when the fish and the polenta are already so the dish can be served hot immediately. And for this we need to peel and chop the garlic and place into the mortar with a generous pinch of sea salt. Make this into a paste, add the water and the vinegar and it's all good to go.
Meanwhile start cooking the polenta, and for this you have to boil the water in a castiron or nonstick pot and add the salt. When the water begins to simmer, add slowly the cornmeal, whisking energetically to avoid lumps. The polenta will start thickening fairly quickly. You will have to keep whisking or stirring fairly often. It may form a crust which is not an issue. In the last minute of cooking stir in the butter and the Parmesan cheese if you choose to use or any other shredded cheese you may like and prefer. Mix until melted and well incorporated.
While the polenta is cooking place a nonstick frying pan on medium heat, add some oil or butter and fry the slightly salted fish fillets. Fry as many as you can fit it in the pan 4-6 at once for about 3 min on each side and then take out on a plate. Repeat with the remainder of the fish fillets until all done.
Once the polenta is ready, dish into four plates, place the pan fried sardine fillets over the soft polenta and drizzle 1 to 2 tablespoons of garlic sauce. Tuck in and fully enjoy. Feel totally free to add some more grated cheese over or even some 1-2 tbsp of yoghurt- a nice full fat Greek style yoghurt or creme fraiche.