Spray square 2-quart baking dish with nonstick spray. In bowl, whisk milk, eggs, mustard, thyme, and 1/4 teaspoon each salt and pepper to blend. Arrange half of bread, overlapping, in baking dish. Pour half of milk mixture over bread; sprinkle with half of Cheddar. Top with ham, then repeat layering of bread, milk and Cheddar. Cover dish; chill 1 hour or up to overnight. Preheat oven to 350 degrees F. Bake 50 to 55 minutes or until golden and custard is set. Let stand 5 minutes before serving. Garnish with chives.