Sheets of spinach-flavored handkerchief pasta enfold low-fat ricotta, broccoli rabe, butternut squash, and shiitake and cremini mushrooms in this variation of traditional lasagna.
Heat oven to 450 degrees. Bring a large stockpot of water to a boil. Trim off 2 to 3 inches of the broccoli rabe, and discard. Place in boiling water; cook 1 to 2 minutes, until bright green and slightly tender. Using a slotted spoon, remove broccoli rabe; transfer to a colander. Place under running cold water until cool; drain.
Combine salt and pepper in a small bowl. In another bowl, place ricotta, lemon zest, and a pinch of salt-and-pepper mixture; stir to combine. Set aside.
Coat a baking pan with olive-oil cooking spray, and arrange broccoli rabe in a single layer. Sprinkle with a pinch of the salt-and-pepper mixture; coat again with cooking spray. Cut butternut squash crosswise into 1/2-inch-thick slices, and place in a large bowl. Add 1 teaspoon olive oil, sprinkle with a pinch of the salt-and-pepper mixture, and toss to combine. Place on a second baking pan; roast until tender and lightly browned, about 35 minutes. After 15 minutes, place broccoli rabe in oven; cook for remaining 20 minutes.
In a low-sided 6-quart saucepan, heat remaining teaspoon olive oil over medium heat. Add mushrooms; cook, stirring, until browned and letting off some liquid, at least 5 minutes. Add 2 cups water and remaining salt and pepper; cook until mushrooms are soft and liquid is reduced by half, 10 to 12 minutes. Combine butter and flour in a small bowl; stir into mushroom mixture. Cook until sauce is slightly thickened, about 2 minutes. Stir in tarragon.
Boil a stockpot of water. Using a fluted pastry wheel, cut pasta sheets in half. Place pasta sheets in boiling water; cook until tender, about 1 minute. Using a slotted spoon, remove from water; place a half sheet on each of 6 plates. Arrange squash, broccoli rabe, and ricotta mixture on each; top with remaining pasta and mushroom sauce. Serve.
Keep vegetables warm in the oven if necessary so all the ingredients are hot when assembled. We used fresh spinach pasta sheets, but any flavor may be substituted. Lasagna noodles may also be used; just be sure to adjust the cooking time.