There are tons of different recipes out there to make stuffing for filled zucchinis. Some say rice, some say beef, some say both. In the end they are all pretty similar and it is really up to your preference is. For our stuffing we used the same combination we use when stuffing bell peppers and just increased the amount we made.
For the filling:
1/2 kg or a little over 1lbs ground beef
2 onions, chopped finely
Crushed tomatoes (here in Germany they are called pizza tomatoes)
2 bulbs garlic, chopped finely (if you love it as much as we do)
Lots of seasoning (oregano, salt, pepper, rosemary...)
2 tsp sugar
Cook the garlic and onions on the stove in a little bit of sunflower oil until the onions begin to look translucent and get brownish. Add the beef and cook through. Add the tomato sauce and seasoning (pepper, salt, oregano, and rosemary). Add about 2 tsp of sugar, or amount to suit your preference (this helps against the very acidic tomato sauce).
While everything is cooking on the stove wash your zucchini and cut in half (long ways). Dig out the middle to make a canal where your filling can go. Our zucchini was huge so we only needed one. If you are using small zucchinis you either need two buy two or make less filling.
Once the filling is cooked and some (but not all!) of the tomato sauce has been cooked off, make sure the taste is to your liking (we like it spicy and added some cholula sauce and chili flakes!). Then spoon it into your zucchini halves. Place on a baking sheet or dish and cook for 20 minutes on 175 C (350 F). You can tell if your zucchini is done by poking it with a fork. The fork should stick into the side quite easily. You do not have to worry about the insides since it was already cooked.
After 20 minutes, take out your zucchini, sprinkle it with shredded cheese and put it back in in the oven until the cheese is melted and slightly brown and a little crusty.