Silky roasted butternut squash stands in for noodles in this more nutritious take on the starchy pasta casserole.
Preheat oven to 350°F. Heat oil in a large skillet over medium-high. Add onion and garlic; sauté 4 minutes. Add spinach; sauté until wilted. Remove from heat. Place yogurt, milk, and provolone in a blender; blend 20 seconds. Add flour, salt, and eggs; blend 1 minute.
Peel neck of the squash. Cut neck from bulb. Cut neck into 24 (1/8-inch-thick) slices using a knife or mandoline. (Use bulb for more slices if needed.)
Place squash slices in an 8-inch square microwave-safe glass baking dish; cover with plastic wrap. Microwave until squash slices are almost tender, about 4 minutes. Remove squash from dish. Coat dish with cooking spray; spread 1/2 cup yogurt mixture in bottom of dish.
Shingle one-third of squash over yogurt mixture. Top with one-third of ricotta cheese, one-third of spinach mixture, and one-third of remaining yogurt mixture. Repeat procedure twice, ending with yogurt mixture.
Sprinkle lasagna with Gruyère. Cover dish with foil; bake at 350°F for 50 minutes. Remove from oven. Turn on broiler with oven rack about 8 inches from top. Remove foil from dish; broil 4 minutes or until lightly browned. Remove from oven; loosely cover, and let stand 20 minutes. Cut into 6 rectangles.