Before you start, line a square tin (about 20x20cm) with baking paper and set aside.
Bring stock to the boil in medium pot. Once boiling, add oregano and a pinch of salt and slowly whisk in polenta, stirring constantly. Cook for 1-2 minutes, until thickened.
Pour polenta into prepared tin and place in fridge to firm up, about an hour.
Preheat oven to 230°C. Slice chilled polenta into chips (about 2x2cm) and place on a lined oven tray. Brush fries with olive oil and season with salt. Bake for 30 minutes, until crisp and golden.
While fried bake, make aioli. Place oil, milk, mustard, salt, garlic and pepper into a nutri bullet (or similar food blender) and fasten on lid. Gently turn the cup upside down and let sit for 30 seconds to allow the liquid to separate into layers. Blend for a few seconds, until thick and creamy. Stir through lemon juice and vinegar and season to taste with salt and pepper.