Preheat your oven to 375°F and line a baking sheet with parchment paper for easy cleanup.
Wash and halve the zucchini lengthwise. Scoop out the centers to create "boats"—leaving about ¼ inch of edge intact. Reserve the scooped flesh for the filling.
Arrange zucchini halves on the baking sheet, lightly mist both sides with cooking spray, and bake cut-side up for 6 minutes. Set aside to cool.
Meanwhile, heat a large skillet over medium-high heat. Add a light mist of cooking spray, then cook the Italian sausage and ground meat along with the reserved zucchini flesh, diced onion, and garlic. Use a spatula to crumble the meat as it cooks, about 8-10 minutes.
Once the meat is fully browned and the onions are soft, stir in the drained tomatoes, spinach, Italian seasoning, salt, and pepper. Continue cooking for another 3-4 minutes until the spinach wilts.
Reduce the heat to low, then stir 1 cup of mozzarella cheese into the filling mixture until combined.
Stuff each zucchini half generously with the filling. Sprinkle the remaining ½ cup of mozzarella over the top.
Return the filled zucchini to the oven and bake for 12-15 minutes until the cheese is melted and bubbling, and the zucchini is fork-tender.
Optional: Garnish with fresh basil just before serving.