Set oven rack at lowest position and preheat to 450 degrees F. Coat a rimmed baking sheet with cooking spray.
Whisk buttermilk and mustard in a medium bowl. Combine cornmeal, flour, cumin, thyme and salt in a shallow dish.
Dip turkey into the buttermilk mixture, then dredge in the cornmeal mixture.
Place the prepared baking sheet in the oven for 5 minutes.
Place the turkey fingers on the hot baking sheet and coat on all sides with cooking spray.
Bake until the bottoms of the fingers are golden, about 10 minutes.
Turn the fingers over and bake until golden brown on the outside and no longer pink in the middle, 8 to 10 minutes more.
While the turkey fingers are baking, prepare maple-mustard sauce: Combine ¼ cup whole-grain mustard and 2 tablespoons maple syrup in a small bowl.
Serve the turkey fingers with the sauce for dipping.