In a saucepan on low heat, melt the butter and add the flour. Whisk to prevent lumps and cook for about 3 minutes. Pour in the milk, a little at a time, whisking constantly. Cook for 10 minutes or until thickened. Season with salt, pepper and nutmeg. Cover and set aside.
Make the crespelle
Crack eggs into a bowl and combine with melted butter and warm milk. Whisk well, then add flour and salt, a little at a time, till a smooth batter forms, with no lumps. Cover with cling film and let stand for about 20 minutes.
Lightly grease a 12-13cm nonstick frying pan and set on low heat. Once hot, pour in one small ladleful of the batter and swirl to spread evenly on the bottom of the pan. Cook until the edges of the crepe start getting dry (1-2 minutes). Then flip the crepe and cook on the other side for about half the time. Transfer the crepe on a plate lined with kitchen paper.
Proceed the same way, making as many crepes your batter will allow. To make it quick, divide the batter between 2 pans and cook in parallel. You'll get about 20 crepes.
Make the filling
Cook the garlic in oil along with the chopped parsley stalks until golden. Add mushrooms and coat well in oil. Season with salt and cook for 10 minutes on medium heat, until mushrooms are soft. Remove from heat, add parsley and discard the garlic.
Assemble and bake
Preheat oven to 180 C / Gas 4.
Grease with butter and sprinkle with Parmesan a 20X30cm baking dish.
Fill each crepe with 1 tablespoon of mushrooms, 1 tablespoon of bechamel sauce and a sprinkle of Parmesan. Fold 4 sides inwards (or just 3, as you prefer) and lay in the baking dish. Proceed filling all the crepes and lay them in the dish slightly overlapping.
Pour the remaining bechamel on the top of the crepes, and spread evenly. Sprinkle with more Parmesan.
Bake in the preheated oven for 20 minutes. Then transfer the dish on the top rack of the oven and grill for 3 to 5 minutes to brown the top. Remove from oven, let stand a few minutes and serve.
This is the perfect dish to make ahead. Prepare the crepes, bechamel and filling the day before and store separately in the fridge. Assemble and bake just before serving.