Combine flour, whey protein, xanthan gum, and salt in a medium bowl. Add butter and cut into flour mixture with a pastry cutter or two knives until mixture resembles coarse meal.
Stir in sour cream until a dough forms.
Transfer dough onto a lightly floured surface. Knead lightly and then roll dough to approximately an 8- x 6-inch rectangle. Fold dough like a letter, folding top third down and bottom third up. Rotate dough 90-degress and repeat rolling and folding. Cover dough with plastic wrap and chill for 30 minutes.
Meanwhile, combine sugar and cinnamon; and set aside.
Place apple slices onto a plate and microwave for 1 minutes or until softened.
Preheat oven to 400°.
Roll dough out on a lightly floured surface to 12- x 10-inches.
Cut dough into 6 (2 - x 10-inch). strips.
Brush with melted butter and sprinkle with cinnamon-sugar mixture.
Place apples slices, slightly overlapping, with peel side facing out onto have of strip. Fold remaining half of dough over apples. Brush with melted butter. Roll up from short side to make rose tart shape.
Repeat with remaining dough, apples, and butter.
Place rolled tarts into muffin tins coated with cooking spray or butter. (sprinkle with additional turbinado sugar, if desired.)
Bake at 400° for 35-40 minutes or until golden brown.