In a large pot over medium-high heat, melt butter. Add onions, thyme and season with 2 teaspoons salt and 1 teaspoons pepper. Cook for 5-6 minutes or until the onions are soft and translucent; add mustard and stir to combine. Add apples and cabbage and stir gently.
Add buttermilk and cream, decrease heat to medium and cover. Let simmer for 10-15 minutes or until cabbage is soft but not mushy. Season with remaining salt and pepper and serve.