Melt 2 Tbsp. butter in a large skillet over medium-high heat; add onion, and sauté 5 minutes or until tender. Add squash, 1 1/2 tsp. salt, and pepper; cover and cook, stirring occasionally, 15 minutes or until squash is tender. Remove from heat; drain well.
Melt remaining 2 Tbsp. butter in a saucepan over medium-high heat; whisk in flour until smooth. Whisk in milk. Bring to a boil; reduce heat, and simmer 2 minutes. Remove from heat; whisk in cheese and remaining 1/2 tsp. salt. Gently stir together squash and cheese mixture in a large bowl. Pour into a lightly greased 11- x 7-inch baking dish. Sprinkle breadcrumbs evenly over top.
Bake at 400° for 20 minutes or until bubbly. Let stand 10 minutes before serving. Garnish, if desired.
Note: To make ahead, prepare recipe as directed; do not top with breadcrumbs. Cover and chill overnight. Remove from refrigerator; let stand 45 minutes. Uncover and top with breadcrumbs; bake as directed.