Heat the avocado oil in a large skillet and add the onion. Sauté, stirring occasionally, until the onion is translucent and softened, about 5 to 8 minutes.
Add the ground beef and allow it to brown for 2 to 3 minutes on both sides before using a spatula to break it into small bits.
Add the remaining ingredients to the skillet and bring to a full boil. Cook, stirring occasionally, until the sauce thickens up, about 10 to 15 minutes.
Preheat the oven to 425 degrees F.
Cut the tips and tails off of each zucchini and use a spoon to scoop out the insides, leaving about ¼ inch of flesh.
Stuff the zucchini boats with the meat mixture, then sprinkle with mozzarella cheese. Bake for 20 to 25 minutes, or until the cheese is golden-brown and melted.
Serve with chopped green onion on top along with any desired side dishes.