Preheat oven to 425 degrees. Coat a 2-quart casserole dish with cooking spray and set aside.
In a large nonstick skillet over medium-high heat, cook the beef for 7 minutes, or until no longer pink, stirring occasionally.
Add the garlic and zucchini, stirring well to mix. Cook for 2 minutes, or until zucchini is tender, stirring occasionally. Add flour, stirring to coat the beef and zucchini.
Add onion soup, ketchup, and water, stirring well to mix, and cook for 3 minutes, or until thickened. Cover and set aside to keep hot.
Meanwhile, heat mashed potatoes according to package directions.
Transfer beef mixture to prepared casserole dish. Spoon potatoes over beef mixture, spreading all the way to the edge of the dish to seal. Sprinkle with the shredded cheese.
Bake for 10 to 15 minutes, until cheese is melted and top is lightly browned.