Mix the potato starch and salt in a bowl, create a well in the middle. Whisk together the milk, water, eggs and butter. Gradually pour the liquid into the well while stirring with a whisk, continue to whisk until the batter is smooth.
To cook the crêpes
Heat a little clarified butter in a non-stick frying pan. Pour 1/4 of the batter into the pan to thinly cover the base. Reduce the heat. Once the underside is cooked and separates from the pan, turn the crêpe, finish cooking, cover and keep warm. Stir the batter well, repeat the step 3 times.
Combine the oil and balsamic, season, set aside 2 tbsp of the dressing. Mix the rest with the spinach.
Fold the crêpes once, fill with Brie, nuts and spinach, fold again, heat for 1 min. in the frying pan, drizzle the reserved dressing over the top, serve warm.