Prepare the Vegetables: Julienne the courgettes, sweet potatoes, and leeks into thin strips. Thinly slice the mushrooms and onion. Cut the spring onions into long strips. Combine all the vegetables in a large bowl, then measure out 4 cups of the mixed vegetables.
Make the Batter: In a separate bowl, combine the flour, cornstarch, and a pinch of salt. Gradually pour in the water while whisking continuously to prevent lumps, creating a smooth batter.
Combine Vegetables and Batter: Pour the batter over the mixed vegetables, stirring until all the vegetables are evenly coated.
Cook the Pancakes: Heat about 2 tablespoons of vegetable oil in a non-stick pan over medium heat.
With these quantities you can make 4 medium ones or more smaller ones. If you make them smaller, you will get a crunchier finish. Spoon a portion of the vegetable mixture into the pan, spreading it out as thinly as possible to ensure a crispy pancake.
Fry for about 5 minutes, or until the bottom is golden and crisp. Check frequently to avoid burning. Carefully lift the pancake with a spatula, shake the pan to ensure it's not sticking, then flip it over. Drizzle a bit more oil around the edges of the pancake, allowing it to run under and crisp up the second side.Cook for another 5 minutes, or until golden and crispy on the other side. Serve: Transfer the pancake to a cutting board and slice it into squares or wedges. Serve hot with the dipping sauce of your choice.