1. Preheat oven to 400°.
2. Combine first 3 ingredients in a large saucepan; bring to a boil. Reduce heat, and simmer 7 minutes or until almost tender. Drain; let stand 5 minutes. Arrange about 1/2 cup vegetable mixture into each of 8 (5 1/2-inch) round gratin dishes coated with cooking spray.
3. Combine milk, broth, flour, salt, and pepper in a saucepan over medium-high heat; bring to a simmer. Cook 4 minutes, stirring constantly with a whisk until thick. Remove from heat; add cheese, stirring with a whisk until smooth. Spoon about 3 tablespoons sauce over each serving.
4. Melt butter in a medium skillet over medium-high heat. Add panko; toast 2 minutes, stirring constantly. Sprinkle breadcrumb mixture evenly over cheese mixture. Place dishes on a baking sheet. Bake at 400° for 15 minutes or until bubbly and golden brown on top. Let stand 5 minutes before serving.