Drop zucchini into boiling water and boil for 5 min. Cool (cooling in cold water will speed the process). Cut in half lengthwise, and scoop out pulp (save pulp).
Place “shells” in glass baking dish (sprayed with non-stick.)
Brown Italian sausage and drain well.
Mix sausage, pulp, vegetables, Italian seasoning, and spaghetti sauce in a medium pan. Bring to a boil. Reduce heat; simmer for 5 min. (Will start out very thick…will thin as simmers.)
Scoop into shells; should be overflowing, smothering them.
In a small pan, melt butter; add flour and salt. Slowly add milk; simmer for 2 min. Add half of parmesan cheese. Whisk in dijon. Pour over filled zucchini shells.