Bring the stock and milk to a boil in a saucepan. Then whisk in cornmeal until smooth. Turn the heat down to medium and cook for 4-8 minutes, stirring frequently.

Remove the saucepan from heat and stir in lemon juice, parmesan, thyme, and salt.

Immediately transfer to a greaseproof paper-lined baking tray. Smooth the surface to about a 2-inch thickness.

Cover with plastic wrap and chill in the fridge for about 2 hours to firm up the polenta.