Butternut Squash and Spinach Lasagna

Silky roasted butternut squash replaces noodles in this more nutritious casserole of starchy pasta.
Preheat the oven to 350°F. Heat the oil in a large skillet over medium-high heat. Add the onion and garlic; simmer for 4 minutes. Add the spinach; saute until wilted. Remove from heat. Whisk the yogurt, milk, and provolone in a blender; whisk for 20 seconds. Add flour, salt and eggs; beat for 1 minute.Peel the pumpkin neck. Cut off the neck of the onion. With a knife or mandolin, cut the neck into 24 1/8-inch thick slices. (If necessary, use the onion for more slices.)Place the pumpkin slices in an 8-inch glass baking dish suitable for a microwave oven; cover with plastic wrap. Cook in the microwave until the pumpkin slices are almost tender, about 4 minutes. Place the pumpkin on a platter. Brush the dish with cooking spray; place 1/2 cup of the yogurt mixture on the bottom of the pan.Sprinkle a third of the pumpkin with the yogurt mixture. Top with a third of the ricotta cheese, a third of the spinach mixture, and a third of the remaining yogurt mixture. Repeat the procedure twice, finishing with the yogurt mixture.Sprinkle the lasagna with gruyere. Cover the dish with foil; bake at 350 ° F for 50 minutes. Remove from the oven. Turn on the grill by placing the oven rack about 8 inches from the top. Remove the foil from the dish; bake for 4 minutes or until lightly browned. Remove from the oven; cover loosely and let stand for 20 minutes. Cut into 6 rectangles.