In a large bowl, combine all ingredients except ketchup, mixing lightly but thoroughly. Place about 1/3 cup of beef mixture into each of 12 medium muffin cups, pressing lightly; spread ketchup evenly over tops.
Bake 20 to 25 minutes, or until no pink remains and juices run clear.
Notes
If you want, you can use yellow squash in place of the zucchini…or some of both!
