Instructions:
Preheat the oven to 350°F.
Spray a 9 x 13-inch oven proof baking dish with vegetable cooking spray and set aside.
In a large skillet, heat the oil over medium heat and add the garlic. Cook until golden, about 2 minutes.
Add the onions, carrots and mushrooms, and cook until the liquid has been released and evaporated, about 5 to 8 minutes.
Toss in the zucchini and summer squash and cook until the liquid has been released and evaporated, about 4 to 5 minutes. Remove from heat, place in a strainer to allow any excess liquid to drain and set aside to cool.
In a large saucepan, whisk together the butter and flour over medium heat. Pour in the milk, whisking vigorously, and continue to cook until thick in consistency, about 15 minutes.
Remove from heat and add the cheese 1/2 cup at a time, stirring until fully incorporated before the next addition.
Place a layer of pasta in the bottom of the baking dish and coat with the marinara sauce. Spread 1/3 of the vegetables over the marinara sauce, then top with 1/4 of the cheese sauce. Repeat two more times (without the marinara), and finish with one last layer of pasta.
Spread one final layer of the cheese sauce over the last layer of pasta, then sprinkle with the parmesan cheese.
Place the lasagna in the oven and bake for 40-45 minutes until heated through, and cheese is browned and bubbly.