Place kohlrabies and carrots in a large saucepan and cover with water. Bring to a boil. Reduce heat; cover and cook for 15-20 minutes or until tender. Drain well; set aside.
In a large skillet, saute onion in 2 tablespoons butter until tender. Stir in the flour, salt and pepper until blended. Gradually whisk in milk. Bring to a boil; cook and stir for 2 minutes or until thickened.
Remove from the heat. Stir in the vegetable mixture, parsley and lemon juice. Transfer to a shallow 2-qt. baking dish coated with cooking spray.
In a small skillet, melt remaining butter over medium heat. Add bread crumbs; cook and stir for 2-3 minutes or until lightly browned. Sprinkle over vegetable mixture.
Bake, uncovered, at 350° for 20-25 minutes or until heated through.