Yes. The lasagna can be completely assembled, covered tightly in plastic wrap, and held in the refrigerator for up to two days prior to baking. Alternatively, you can freeze uncooked lasagna for three months prior to baking in a freezer-safe dish tightly wrapped in plastic. Defrost it in the refrigerator overnight. Then, remove the wrap and bake as per the recipe instructions. If it is not completely thawed, it will likely take longer than the recipe calls for to bake properly.   You can also bake the lasagna from its frozen state. In this case, add about 20 to 30 minutes to the baking time and cover it with foil for the first 30 minutes of baking.   Also, the béchamel and mushroom/spinach mixture can be prepared up to two days in advance and held in airtight containers in the refrigerator. If you do this, I suggest (1) slowly and carefully heating up the white sauce over low heat prior to assembly so that it is easier to spread, and (2) straining out any residual liquid that has accumulated in the veggie mixture.