In small saucepan, combine garlic, ½ cup milk and oil. Cover, and simmer over medium-low heat until garlic is tender, about 20 minutes. Remove from heat and set aside.
Meanwhile, put potatoes into large saucepan, and cover with cold water. Bring to a boil, reduce heat, and simmer until potatoes are tender, about 20 minutes. Drain, and set aside.
Lightly coat ovenproof casserole with cooking spray. In large saucepan, combine ½ cup water with cabbage, leeks and cauliflower. Cook over medium heat until water evaporates and vegetables are crisp-tender, about 15 minutes. Sprinkle vegetables with flour and stir over low heat, 2 minutes. Remove pan from heat.
Gradually add remaining 1 cup milk, stirring until well blended. Return pan to heat, and stir until sauce thickens. Add cheese slices, and stir until cheese is melted, and sauce is smooth. Spoon cabbage-leek mixture into prepared casserole dish.