1. Tie thyme sprigs with twine or in a cheesecloth bag. Combine milk, garlic and thyme in a small saucepan. Bring to a simmer over medium heat. Reduce heat to low and simmer gently until garlic is soft (5-10 minutes). Discard thyme.
2. Transfer the milk mixture to a blender and puree until smooth. Return to the pan and stir in scallions. Cover and set aside.
3. Bring water to a boil in a large pot. Add cabbage, cover and cook until very tender (8-10 minutes). Drain well.
4. Meanwhile, place potatoes in a large saucepan and cover with cold water. Bring to a boil, reduce heat to medium and cook, covered, until tender (10-15 minutes).
5. Drain the potatoes and return to the pan. Mash, gradually stirring the milk mixture into the potatoes until smooth. Stir in cabbage and season with salt and pepper. Serve warm, garnished with parsley.