Preheat oven to 450°F and line baking sheet with parchment paper.
Mix mustard powder, rosemary, garlic powder, red pepper flakes, and black pepper, if using. Taste and adjust seasonings as desired.
Add potato wedges to a large bowl. Add dry seasonings and toss to coat.
Place potatoes in a single layer on parchment paper. Bake 50 minutes, flipping halfway through and cooking until potatoes are crispy and golden brown.
Meanwhile, prepare Maple Mustard Sauce by whisking together cashew butter, mustard and maple syrup. Add nut milk to achieve desired consistency. Add black pepper to taste.
Remove fries from oven and serve with Maple Mustard Sauce.