Heat your oven to 100°C/Fan 80°C/Gas ½. Beat the egg in a large bowl. Pour in 300ml of the milk. Add in the flour, saving 1 tbsp for later. Season and whisk to make a batter about as thick as double cream. Cover with a tea towel and set aside to rest.
Peel and slice the onion. Slice the mushrooms, or halve or quarter them if they’re very small. Coarsely grate half the piece of Cheddar.
Heat a large pan for 1 min. Add ½ tbsp oil and the onion. Season with salt and pepper. Cook and stir for 2 mins to brown a little. Add the mushrooms. Cook and for 3 mins till soft and juicy.
Sprinkle in the reserved flour and stir. Cook for 1 min. Slowly pour in the remaining milk. Stir till you have a thick sauce. Sprinkle in half the grated Cheddar. Stir and season. Turn off the heat. Cover and set aside.
Place a large frying pan over a medium heat. Add 1 tsp oil and swirl it in the pan to coat it. When the pan is hot, pour in a quarter of the batter. Move the pan in circles to coat the base of the pan. Cook for 2 mins till bubbles appear on surface and the pancake is browned underneath.
Flip the pancake over and cook for 1 min. Slide the pancake onto a plate and place in the oven to keep warm while you repeat with the remaining batter to make 4 pancakes in total.
Place a pancake on a plate. Spoon over some creamy cheesy mushrooms. Add a few slices of Milano salami. Scatter over some rocket and chervil leaves. Sprinkle over some of the remaining Cheddar. Fold the pancake over. Repeat with the other 4 and serve.