Preheat oven to 375 degrees. Brown meatballs in a large skillet over medium-high heat (about 2 minutes per side). (To bake meatballs instead: place meatballs on aluminum foil-lined baking sheet. Bake 15 minutes; gently turn meatballs over and bake an additional 13 to 17 minutes until internal temperature reaches at least 165ºF.) Meanwhile, chop zucchini into cubes (about ½ inch size) and slice mushrooms.
Reduce heat to medium-low then add zucchini, mushrooms, and sauce and cook until bubbly (about 3 minutes). Turn off heat then stir in spinach.
Break spaghetti noodles in halves and add them to a large, non-stick baking dish then pour sauce and meatballs over top to cover the noodles. Cover dish tightly with foil and bake for about 25 minutes.
Remove foil and, using a fork, stir to mix pasta with sauce (Pasta will still be slightly stiff) then pour in ½ cup water. Re-cover and bake an additional 5-8 minutes or until no water remains in pan.
Sprinkle mozzarella cheese over top before serving.