Heat a large skillet over medium-high heat.
Cook ground beef and diced onion until browned and the onion is soft, about 5–7 minutes. Drain excess fat.
Crush the stewed tomatoes slightly, then add them to the skillet.
Stir in mushrooms, tomato sauce, olives, garlic, salt, and pepper.
Simmer uncovered for 20–30 minutes until the mixture thickens.
Slice zucchini lengthwise, then cut into ½-inch pieces.
Add zucchini to the skillet, cover, and cook for 5–7 minutes until tender but still slightly firm.
Stir in shredded cheddar cheese.
Cover and cook until cheese melts, about 3–4 minutes.
This Zucchini and Ground Beef Skillet is best served warm right out of the pan.