Heat oven to 350°F. Spray 13x9-inch (3-quart) glass baking dish with cooking spray.
In 4-quart Dutch oven, melt 2 tablespoons of the butter over medium-high heat. Cook onion in butter 3 minutes, stirring occasionally. Add squash; cook 10 minutes, stirring frequently, until tender. Remove from heat; stir in sour cream, cheese, soup mix, eggs and salt. Spread in baking dish.
In medium microwavable bowl, microwave remaining 1/4 cup butter covered on High 30 seconds or until melted. Stir in crushed crackers. Sprinkle over casserole.
Bake uncovered 40 minutes or until bubbly and set. Let stand 10 minutes before serving.