Preheat oven to 375 degrees F. Spray a baking pan with nonstick cooking spray. Use one large enough to fit all the squash halves.
Cut yellow squash in half lengthwise. Hollow out each half with a spoon. If the seeds are large, discard them. If not, dice up the squash you removed and add it to your ground beef filling below.
Season squash halves generously with salt, pepper and garlic powder. Arrange in sprayed baking dish.
Brown ground beef and onion; drain excess grease if needed. Season generously with salt and pepper.
Combine browned ground beef, cracker crumbs, egg, both cheeses, and Worcestershire sauce. Add diced up squash if you have it.
Mound the ground beef filling into the seasoned squash halves.
Add a small pat of butter to the top of each mounded squash.
Cover with foil and bake for 30-45 minutes or until a knife cuts through the squash easily.
Remove the foil, and bake for an additional 10-15 minutes or until browned.