For the mushroom crepes: Preheat the oven to 400 degrees F.
Add the milk, flour, mushroom powder, eggs and salt to a blender. Blend to a thin batter consistency.
Spray a nonstick skillet or crepe pan with vegetable spray and place over medium heat. Ladle 1/4 to 1/2 cup batter into the pan. Swirl around so the batter evenly coats the bottom and slightly up the sides. Cook until the crepe is dry on the surface, 1 to 2 minutes, then use a wooden skewer to loosen the edges. Gently move the skewer under the crepe to flip it. Cook on second side for about 30 seconds. Remove to a sheet pan. Repeat with the rest of the batter.
For the mushroom filling: Add the butter to a large skillet over medium-high heat. Cook until the butter melts and the foam has subsided, then add the mushrooms and shallots and saute until soft, 2 to 3 minutes. Add the garlic and thyme and saute another 1 to 2 minutes. Season with a big pinch of salt and a few grinds of pepper. Stir and saute until the liquid evaporates and mushrooms are lightly caramelized, 6 to 7 minutes. Place the skillet in the oven to roast the mushrooms, 8 to 10 minutes.
Remove the skillet from the oven and deglaze with the sherry vinegar. Remove to a bowl, let cool slightly and then fold in the creme fraiche and parsley. Season to taste with salt and pepper. Set aside.
Reduce the oven temperature to 225 degrees F.
For the mushroom hay: Separate the enoki mushrooms and place on a lined baking sheet in an even layer. Place in the oven to dry out, around 1 hour.
Heat the oil to 375 degrees F in a large Dutch oven or high-sided pot. Using a slotted spoon, quickly fry the enoki mushrooms for just a few seconds. Remove and drain on paper towels. Season with salt.
For the mustard sauce: Mix together the mustard and soy sauce in a small bowl. Place a dollop in the center of a serving dish.
Place a crepe on a flat surface. Spread about 3 tablespoons filling down the middle. Fold the sides around the filling (like folding a letter). Flip over and place on top of the mustard sauce, seam-side down. Garnish with the mushroom hay, chervil and more dots of mustard sauce on the plate. Serve. Repeat to make more filled crepes.