Whisk together the mustard, sour cream, aioli and Guinness in a small bowl. Set aside.
Preheat oven to 400° F. Place oven rack in middle of oven. Toss potatoes with oil, salt and pepper to taste until well coated. Spread out in a single layer on a baking sheet. Bake 35-45 minutes or until browned and crisp.
Wrap tortillas in foil and heat in oven until warm, 5-10 minutes.
Combine cabbage and kale.
Serve tacos topped with a drizzle of mustard sauce and chopped parsley.