Preheat oven to 350°.
Place potato in a large Dutch oven, and cover with water. Bring potato to a boil, and cook for 6 minutes or until tender. Remove potato with a slotted spoon, reserving cooking liquid in pan. Place potato in a large bowl, and set aside. Add cabbage to the cooking liquid in pan, and cook for 5 minutes. Drain well. Add the cabbage to potato.
Melt the butter in a small skillet over medium heat; add sage and garlic. Cook for 1 minute, stirring constantly. Stir the chopped sage mixture, salt, and pepper into potato mixture. Spoon potato mixture into a 2 1/2-quart casserole dish coated with cooking spray.
Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, milk, and remaining ingredients, and stir with a whisk. Pour milk mixture over potato mixture (do not stir). Bake at 350° for 50 minutes or until casserole is lightly browned.