Melt 2 tablespoons of butter in a large pot over medium heat. Add the diced onion and sliced leek. Sauté for about 5 minutes until soft and translucent.

Stir in 4 cups of diced starchy potatoes and cook for an additional 3 minutes.

Add in 6 cups of vegetable broth and bring to a gentle boil. Reduce the heat to low and allow to simmer.

Add 2 cups each of chopped green cabbage and kale to the pot. Cover and simmer for 15 to 20 minutes, until the potatoes are fork-tender.

Use an immersion blender to puree the soup to your desired smoothness, blending for about 2 minutes.

Gently stir in 1 cup of milk, and season the soup with salt, pepper, and nutmeg. Heat over low for another 5 minutes.

Ladle the soup into bowls and garnish with sliced green onions.