Bring the vegetable stock to a boil in a large pan.
Whisk the polenta into the stock in a thin stream until very thick.
Stir in grated Parmesan and season generously with salt and pepper.
Spread the polenta onto an oiled baking tray to 1cm thickness.
Refrigerate the tray for one hour until the polenta is firm.
Sauté sliced onions and thyme in olive oil over low heat for 30 minutes.
Add balsamic vinegar and sugar, cooking until the liquid evaporates.
Heat the grill and cut 20 circles of polenta using a 5cm cutter.
Place polenta rounds on oiled sheets and top with onions and Taleggio cubes.
Grill for 3-4 minutes until the cheese is melted and golden.
Transfer to plates and garnish with extra thyme sprigs.