Preheat oven to 350°. Lightly oil a large baking sheet and arrange squash slices on it (they will overlap). Roast until tender, about 15 minutes. Set aside to cool.
In a small bowl, whisk together 3 tbsp. olive oil, vinegar, lemon juice, garlic, salt, and orange zest.
In a serving dish, layer squash slices, red onion, and parsley. Drizzle with dressing and top with parmesan if you like.
Note: Nutritional analysis is per serving.