Preheat oven to 400℉
Slice zucchini into very thin slices. Line your baking pan with parchment paper and brush with olive oil. Then spread the zucchini slices on oiled parchment paper and sprinkle with salt and pepper. Roast about 8-10 minutes. Watch carefully so they do not brown.
Turn oven temperature down to 350℉
Meanwhile, slip each sausage out of its casing, add a little water to your skillet and cook for about 8-10 minutes. Crumble up the sausage with a spatula while it sautés. Spoon into a bowl and set aside.
Season ground beef with salt. Brown the beef in a pan, drain off about half the fat, and then add one cup of sauce, stir and let it simmer for a minute or two.
Spoon sauce into bottom of your casserole dish. Layer with zucchini strips, the ground beef mixture, one and a half cups of cheese, zucchini strips, sausage, one cup of sauce, the remaining one and a half cups of cheese and a final layer of zucchini strips.
Bake uncovered for 30 minutes. Serve immediately with extra sauce and grated parmesan cheese, if you like.