Microwave butternut squash at HIGH 2 minutes (This step softens squash for slicing). Cut squash in half lengthwise; remove and discard seeds. Peel squash, and cut into 1/2" cubes. Set aside 3 cups cubed squash; reserve any remaining squash for another use.
Combine 3 cups squash, sweet potato, and 2 tablespoons olive oil on a large rimmed baking sheet. Bake at 450° for 10 minutes.
Meanwhile, combine leeks, bell pepper, and remaining 1 tablespoon oil in a large bowl. Add to partially roasted squash mixture, stirring gently. Bake at 450° for 20 minutes or until vegetables are tender, stirring after 15 minutes. Return roasted vegetables to bowl; set aside.
Combine milk and garlic in a large saucepan; bring just to a boil. Reduce heat, and simmer, uncovered, 10 minutes. Remove and discard garlic.
Melt butter in a large saucepan over medium heat; whisk in flour until smooth. Cook 1 minute, whisking constantly. Gradually whisk in warm milk; cook over medium-high heat, whisking constantly, 12 to 13 minutes or until slightly thickened. Remove from heat; stir in salt and pepper. Add to roasted vegetables, stirring gently.
Spoon 1 cup vegetable mixture into a lightly greased 13" x 9" baking dish. Top with 3 lasagna noodles; spread half of remaining vegetable mixture over noodles, and sprinkle with 1/2 cup Asiago cheese. Repeat procedure with 3 noodles, remaining vegetable mixture, and remaining Asiago cheese. Break remaining 3 noodles in half and lay on top of casserole. (Breaking the noodles keeps them from curling up.)
Beat cream at high speed with an electric mixer until soft peaks form. Spread whipped cream over noodles; sprinkle with Parmesan cheese. Bake, covered, at 350° for 30 minutes. Uncover and bake 13 more minutes or until golden and bubbly. Let stand 15 minutes before serving.
Made this recipe for Thanksgiving; we have 3 vegetarians, and it was very well received by everyone. Good warmed over too. A little labor intensive so allow plenty of time. I substituted no boil lasagna noodles. I also baked for 45 minutes before uncovering but maybe I didn't let the sauce thicken enough before putting it all together. Served easily and neatly when cut into square portions.
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