To make your own tostada shells, heat 1/2 inch of safflower oil to 350 degrees in a straight-sided skillet. Fry corn tortillas, turning them every 30 seconds, until crisp and golden, about 2 minutes total. Drain on paper towels; sprinkle with coarse salt.
You can roast the squash and prepare the bechamel and filling (omitting the egg) up to two days before assembling the lasagna; let each cool before refrigerating in separate containers. Stir the egg into the filling just before using.
A paillard is simply a skinless, boneless chicken-breast half that you butterfly and pound thin. It's one of the speediest routes to a lean meal: While you prep and saute it, roast some squash and red onion, then place it on top with fresh spinach and crisp fried sage.