Start the filling: Heat the oven to 425°F and arrange a rack in the middle. Cut the squash in half lengthwise and scrape out the seeds. Brush the flesh with oil and season generously with salt and pepper. Place the squash halves cut-side down on a baking sheet and roast until fork-tender, about 35-45  minutes. ( You could do this ahead.)

Make the crepes: In a large mixing bowl, whisk together the flour and the eggs. Gradually add in the milk and water, stirring to combine. Add the salt and melted butter; beat until smooth.  Heat a lightly oiled or buttered 8-10 -inch frying pan over medium-high heat. Pour or scoop the batter onto the pan using approximately 3 tablespoons batter for each crepe. Tilt the pan with a circular motion so that the batter coats the surface evenly. Cook the crepe for about 2 minutes, until the bottom is light brown. Loosen with a spatula, turn and cook the other side. Set aside and stack. ( you could make these ahead and refrigerate, covering)

3. Make the Mushroom Topping: Saute the shallot and mushrooms in butter and oil over medium heat seasoning with salt and pepper and fresh sage, cooking until tender and golden. Drizzle with a teaspoon of truffle oil, if you like. Set aside.

4. Make the Bechamel Sauce. Melt butter in a small, heavy saucepan over medium heat until foaming. Add shallots (if using) and sauté 2 minutes. Do not let brown. Reduce heat to low, add flour, and whisk until smooth and raw taste is cooked off, about 1 minute. Gradually whisk in milk, starting with ½ cup, whisking well, then adding another half cup at a time until all is incorporated. Cook until just thickened, stirring often, about 10 minutes. Stir in nutmeg and salt. Season with ground white pepper. ( You could make this ahead, refrigerate, and heat up ( whisking) before serving. Loosen, if needed with a little milk or water.)

5. Make the Filling: Remove the squash from the oven and let sit at room temperature until cool enough to handle, about 10 minutes. Scrape the flesh with a fork to make long strands; place in a bowl. You should have about 4 cups. If there is more, save the extra for another use. Add nutmeg, maple syrup, white pepper, sage, sautéed onions (optional) and cheese and mix thoroughly, and taste for salt, adding more to taste. (You could make this ahead.)

Assemble: Divide the squash filling among the crepes, and fold them over. ( You could fill these ahead and refrigerate.) Heat  ½ T butter and ½ T olive oil in a pan or skillet and fry each stuffed crepe on both sides until golden and crispy, adding more butter if necessary, and placing in a warm oven until all are crisped.

To plate, place two crispy crepes on top of each other and cut down the middle, so you have 4 triangles. Stack vertically on a plate and drizzle with béchamel sauce, top with sautéed mushrooms. Serve immediately.