At the same time, in another similar sized pot, MELT the butter and whisk in the flour and cook for 1-2 minutes.
Slowly WHISK the warm milk into the roux, bring to a simmer and season with salt and pepper.
REMOVE from heat and stir in 3/4 cup Parmigiano-Reggiano cheese.
Meanwhile, COOK the beef until slightly brown, add the marinara sauce and 1/2 jar of water, season with salt and black pepper and simmer until it is reduced in a large nonstick skillet.
SPRAY a 13 x 9 inch glass baking pan with cooking spray. On the bottom of the pan, PLACE 1/6 of the béchamel sauce and cover with 3 sheets of no-boil lasagne.
TOP with 1/6 of the béchamel and 1/5 of the meat sauce and continue alternating with the lasagne sheets for 4 more layers. The final layer should be béchamel topped with sauce and the remaining 1/4 cup of Parmigiano-Reggiano cheese.
BAKE for 20 minutes covered with foil, then uncovered for five more minutes or until the corners are brown and bubbling.