Set Sun Oven out to preheat. Spray a 9-inch square baking dish with cooking spray.
Bring 3 cups of water to boil in a sauce pan. Add the asparagus and cook for 2 minutes. Drain and set aside to cool.
Whisk together the milk, cornstarch, flour, butter, salt, and pepper in a saucepan over medium heat. Cook, whisking constantly, until the sauce thickens and comes to a boil, about 5 minutes. Remove from heat, stir in the green onions and ¼ cup of the Parmesan cheese.
Spread ¼ of the sauce into the prepared baking dish; top with 2 lasagna noodles, then spread ⅓ of the ricotta and ⅓ of the asparagus over the noodles. Repeat the layers two times with the sauce, noodles, ricotta, and asparagus. End with the last 2 noodles and the remaining sauce.
Cover the dish with aluminum foil then cover with a dark tea towel and bake the lasagna in the Sun Oven until heated through and noodles are soft.
Remove the tea towel and the foil, sprinkle with the remaining Parmesan cheese and bake until the cheese is bubbling, about 2o minutes longer.
Let stand 10 minutes before serving.