Slowly cook in a pan with very little water the asparagus: water must be just enought to cover the vegetables. You want your asparagus to be cooked but still quite firm, keeping a nice green color.
When the asparagus are cooked, throw them in the blender with a splash of milk and some grated cheese (but save some cheese for later use).
When the asparagus cream is ready, it's time to get ready "build" lasagna. Take a pan and spray it with non-stick cooking spray: spread 1 cup of sauce on the bottom of the baking pan and begin layering.
Layering simply means, in this case, placing a sheet of lasagna, then bechamel, then asparagus cream and so on until you finish the ingredients. The lasagne will expand to the edges of the pan during cooking, so make sure there's enough cream everywhere, spreading the filling to the edges of the pan to seal in and cook the lasagne during baking.
Top your lasagna with the remaining grated cheese and place in the oven, 180°, for an hour, or until the lasagna looks ready!