Start by thickly slicing all of the vegetables eggplant, tomatoes, onion & squash - about ½ inch slices, set aside.
Take your skillet or casserole pan and pour about half of the jar of spaghetti sauce into the bottom of the pan, covering it completely.
Begin assembling the casserole starting with the largest vegetables first - layer the eggplant around the side, then tomatoes, onion and finish with the squash. If there are any extra vegetables, tuck them in the edges between the skillet & eggplant.
Then slice the mozzarella cheese and tuck that in between the vegetables. Sprinkle the vegetables generously with salt & pepper. Pour the remaining spaghetti sauce over the casserole dish.
Spread the bread crumbs over the top of the vegetables, sprinkle with olive oil. Cover tightly with aluminum foil and place in the oven for 45 minutes. Remove the foil and bake an additional 15 minutes until the bread crumbs are browned.
Remove from the oven and allow to sit about 10 minutes before serving.