Bring 1 inch of water to a boil in a Dutch oven. Place potatoes in a steamer basket and steam, covered, until just cooked through, about 15 minutes. Transfer to a large bowl and cover to keep warm.
Meanwhile, heat butter in a large nonstick skillet over medium heat. Add onion and cook until translucent, about 2 minutes. Add cabbage and continue cooking, stirring occasionally, until the cabbage begins to brown, about 5 minutes.
Reduce heat to low. Stir in milk, salt and white pepper; cover and cook until the cabbage is tender, about 8 minutes. Add the cabbage mixture to the potatoes. Mash with a potato masher or a large fork to desired consistency.
1 starch, 2 vegetable, 1/2 low-fat milk