Melt the butter in a small saucepan set over medium-low heat. Whisk in the flour and cook 3-4 minutes, or until light golden brown. Very slowly whisk in the milk. Bring to a simmer and cook 5 minutes. Season with salt, pepper, and nutmeg.
Heat the oil in a 5 quart sauté pan set over medium heat. Add the onion and cook for about 8 minutes, or until translucent. Add the garlic and cook 1 minute. Stir in the vegetables and red pepper flakes; cook 5-6 minutes, or until the vegetables begin to soften. Season with salt. Add the lasagna to the pan; pour the sauce over the top.
Cover and cook 25 minutes, stirring occasionally, until the pasta is tender. Drop tablespoons of ricotta over the top of the lasagna, then cover with shredded mozzarella. Cover and cook 5 minutes, or until the cheese is melted. Remove from heat and top with fresh basil.
Let the lasagna sit for 5-10 minutes before serving.