1Heat oven to 375°F. Generously spray 8-inch square (2-quart) glass baking dish with cooking spray.
2In 12-inch skillet, melt butter over medium-high heat. Cook onion in butter 2 minutes, stirring frequently, until almost tender. Add squash; cook 4 minutes, stirring occasionally, until just tender and lightly browned. Remove from heat; cool 5 minutes.
3In large bowl, beat eggs with whisk until blended. Stir in half-and-half, 3/4 cup of the cheese, the thyme leaves, salt and pepper. Gently stir in squash mixture and bread cubes. Spoon into baking dish.
4Bake uncovered 20 minutes. Top with remaining 1/2 cup cheese; bake 10 minutes longer or until puffed and golden. Let stand 10 minutes before serving. Garnish with thyme sprig.