Step 1Put squash, sour cream, cheese, egg, 3 tablespoons onion (reserve rest for topping), garlic, 1/2 teaspoon salt and pepper to taste in bowl of food processor and mix until smooth, 10 seconds. Transfer to greased 5-cup shallow baking dish. Smooth top.
Step 2Toss together bread crumbs, butter, reserved onion and remaining salt. Adjust seasonings. Spread evenly over squash. (Can be made 1 day ahead and refrigerated. Let come to room temperature before baking.)
Step 3Bake at 350 degrees, uncovered, until edges are just beginning to brown and crumbs are golden, about 40 minutes. Serve hot.