Line a large baking dish with parchment paper and drizzle with olive oil — You can skip the parchment and just coat with oil if preferred! Prep ingredients and toppings. For the zucchinis, cut in half lengthwise and hollow out using a spoon or ice cream scoop. Arrange zucchini in prepared baking dish, drizzle with olive oil and sprinkle with salt, pepper, and Italian seasoning. Set aside.
Preheat oven to 400 degrees. In a large pan, heat olive oil over medium, add garlic and onions and sauté. Add the sausage and beef to the pan, season with salt and pepper, and break up the meat as it cooks, until cooked through.
Add marinara sauce to pan and let simmer for 5 minutes. Spoon mixture into zucchini boats, top with cheese, and bake in preheated oven for 25 minutes.
Remove from oven, let cool for 5 minutes, and serve with prepared toppings!