This vegetable lasagna with white sauce has a velvety sauce with lots of veggies and plenty of cheesy goodness. The layers of butternut squash give the dish a sweet and creamy flavor and texture. This vegetarian lasagna packs comfort in every bite.
Preheat oven to 375°F. Lightly coat a 9-by-13-inch baking dish with cooking spray.
Heat oil in a medium skillet over medium-high heat. Add onion and cook, stirring often, until just tender, about 5 minutes. Add garlic and cook, stirring often, for 1 minute. Transfer the onion and garlic to a large bowl; stir in frozen vegetables and basil.
Melt butter in the skillet over medium-high heat. Add flour and whisk to combine. Cook, whisking, for 30 seconds. Gradually add milk; cook, whisking constantly, until the mixture thickens, 2 to 3 minutes. Add Parmesan and salt; whisk until smooth. Remove from heat.
Spread 1/2 cup of the cheese sauce on the bottom of the prepared baking dish. Layer on 3 lasagna noodles, evenly spacing the noodles crosswise in the dish. (The noodles will not cover the bottom of the dish but will expand when baking.) Top the noodles evenly with 3 cups vegetable mixture, 3/4 cup cheese sauce and 3/4 cup shredded cheese. Layer 3 lasagna noodles over the cheese; spread half the squash over the noodles and sprinkle with 3/4 cup cheese. Repeat the layers once, starting with noodles and ending with squash. Sprinkle with 3/4 cup cheese. Layer with the remaining 3 lasagna noodles; spread the remaining cheese sauce over the noodles and sprinkle with the remaining cheese.
Lightly coat a sheet of foil with cooking spray; cover the baking dish coated-side down. Bake for 30 minutes. Uncover and bake until golden brown and bubbly, about 15 minutes more. Let stand for 15 minutes before serving. Sprinkle with basil, if desired.