Preheat oven to 350°F. Prepare stuffing: In a large sauté pan on medium-high, heat 1 tsp oil. Add garlic, onion and mushrooms. Sauté until mushrooms soften, about 10 minutes. Remove from heat and add remaining 2 tsp oil, vinegar, bread crumbs and Parmesan. Mix thoroughly.
Spoon an equal amount of stuffing into each zucchini half. Bake on a foil-lined cookie sheet for 10 minutes.