Melt 1 tbsp butter in a large saucepan over medium heat. Add potato, onion, 1/4 tsp salt & 1/4 tsp pepper. Cover & cook 6 minutes.
Add broth and 2 cups water; bring to a boil. Cook 10 minutes or until potato is tender.
Combine 3 tbsp water and remaining 1 tbsp butter in a large Dutch oven; bring to a simmer. Add cabbage and thyme. Cover and cook 5 minutes, stirring occasionally. Remove from heat; stir in remaining 1/4 tsp salt & 1/4 tsp pepper.
Place 1/2 of potato mixture in blender. Remove center piece of blender lid to allow steam to escape. Place lid on blender and cover with paper towel to avoid splatters. Blend until smooth. Pour into large bowl. Repeat process with remaining potato mixture.
Add potato mixture to cabbage mixture; cook over medium-low heat until thoroughly heated.