Preheat oven to 350 degrees. Heat oil in a large skillet, add onion and stir over medium heat until tender and transparent.
Wash zucchini, discard ends, and cut crosswise into 1/4 inch slices. Add to onion and stir for 3 to 4 minutes over medium heat. Turn into a 2-quart casserole.
Crumble ground beef into the skillet, stir over medium heat and brown until no longer pink. Drain well.
Add salt, garlic and oregano to tomatoes in large bowl and mix thoroughly. Add beef and mix well. Add meat mixture to zucchini in casserole. Mix together gently. Sprinkle with cheese. Bake 30 to 35 minutes until zucchini is tender.