1. Place kohlrabi and carrots in a large saucepan; cover with water. Bring to a boil. Reduce heat; cover and cook for 15–20 minutes or until tender. Drain well; set aside. (This can be done earlier in the day.)
2. In a large skillet, sauté onion in one and a half tablespoons oil until tender. Stir in the flour, salt, and pepper until blended. Gradually whisk in soy milk. Bring to a boil; cook and stir for two to three minutes or until thickened. Remove from heat and stir in the vegetables, parsley, and lemon juice. Transfer to a 2-quart oven-to-tableware dish coated with oil spray.
3. Wipe out the skillet with a paper towel, then heat remaining oil in it. Add bread crumbs and stir for two to four minutes or until crispy. Sprinkle over casserole. Baked uncovered at 350 degrees Fahrenheit (180 degrees Celsius) for 25 minutes or until set.