Preheat oven to 375 degrees Fahrenheit. Line a 13x9 baking dish with parchment paper, or grease with olive oil.
Slice zucchinis in half lengthwise. Be sure to keep the cuts uniform. With a spoon, scrape out the zucchini pulp and set aside. Place the hollowed zucchini halves into your baking dish.
Place a large skillet over medium heat on the stovetop and add the olive oil. Finely chop the onion and garlic.
Add the onions to the skillet and cook down until just beginning to turn translucent, for about 2-3 minutes. Add the garlic to the skillet and sauté until fragrant, about 1-2 minutes.
Add the ground beef and Italian sausage to the skillet, breaking up larger chunks with the back of your wooden spoon. Cook until there is no pink left.
Add the tomato sauce, zucchini pulp, Italian seasoning, salt, and pepper to the skillet. Simmer for 10 minutes.
Once the mixture has reduced and smells delightful (it will, seriously), fill your zucchini halves with the mixture.
Bake for 18-20 minutes to soften the zucchini halves.
Remove from the oven, and add heaping scoops of Parmesan cheese to the top of each zucchini boat.