Line a 25cm x 16cm (or thereabouts) rectangular oven tray with baking paper.
Put your polenta into a jug (this makes it easier to add to the pot). Put the milk, stock and salt into a medium saucepan and heat until almost boiling.
Get your whisk ready and pour the polenta into the hot milk/stock mixture in a steady stream while constantly whisking away. This helps prevent any lumps. Keep stirring until the polenta is cooked and thickened – about 3 minutes.
Remove from heat, stir in the Parmesan and then pour onto the prepared baking tray.
Refrigerate until set (1–2 hours). You can do this step the day before to make things easier.
Preheat oven to 200°C (390°F).
Turn the set polenta out onto a cutting board and cut into chip shapes – go with whatever size you like here. I like them about 10cm x 2cm, but there's no need for measuring.
Place 'chips' onto lined baking trays and brush with a little olive oil (or spray with olive oil spray). Bake for 25–30 minutes. They’re ready when the edges look crispy and charred.
Remove from the oven. Sprinkle with rosemary and sea salt. Serve hot.
