Preheat oven to 375°F; grease a 9x13 inch dish with cooking spray or butter.
Toss squash with oil, salt, and pepper; spread on baking sheet and roast 25 minutes until tender and lightly caramelized.
Meanwhile, sauté onion in large skillet over medium 5-6 minutes until softened; add garlic and cook another minute.
Gently mix squash, onion, garlic, 1 1/2 cups mozzarella, Parmesan, cream, thyme, and sage in large bowl until combined.
Spread evenly in dish; top with remaining 1/2 cup mozzarella.
Mix panko and melted butter in small bowl; sprinkle evenly over casserole. Bake 25-30 minutes until top is golden brown and edges bubble.