- Bring a large pot of salted water to a boil. Using a sharp paring knife, cut the core out of the cabbage. Carefully lower the whole head of cabbage into the boiling water, and cook for 4 to 5 minutes, until outer leaves begin to loosen. Remove cabbage from water and carefully peel off 8 outer leaves. Reserve remaining cabbage for another use.
- In a large skillet, heat coconut oil and sauté mushrooms, carrots, and parsnips until tender, 4 to 5 minutes. Add garlic and cook for 1 minute longer, stirring. Stir in walnuts. Remove from heat and let cool slightly.
- To assemble, place one cabbage leaf on a flat surface. Mound a few spoonfuls of filling in the center of leaf. Fold bottom of leaf over filling, and roll up about 1/3 of the way. Fold sides in and continue rolling into a tight roll. Place on a platter, seam side down. Repeat with remaining rolls and filling.
- To make sauce: Whisk together mustard, honey or agave, walnut oil, and tarragon in a small bowl. Season to taste with salt and pepper. Drizzle over cabbage rolls, shower with parsley, and serve.