Preheat oven to 400 degrees F. Line a baking sheet with parchment paper.
Melt butter in a large saute pan over medium heat. Add chopped onion and apple and cook until softened, about 4-5 minutes. Remove from heat and stir in 2 tablespoons maple syrup. Set aside half and let cool.
With the remaining half make the apple mustard sauce. Combine the sauteed apples and onion in a blender. Add stone ground mustard, Dijon mustard, maple syrup, and apple cider vinegar. Puree until smooth. Set aside.
Once cooled, combine sauteed apples and onion with ground turkey, breadcrumbs, egg, thyme, garlic powder, salt, pepper, sage, fennel, and nutmeg in a large bowl. Mix until well combined. Divide turkey filling into 4 portions.
Cut the 2 sheets of puff pastry in half lengthwise, making 4 long strips. Shape the turkey filling into logs the same length as the puff pastry and place filling towards the bottom of the pastry. Brush egg wash along the edges and roll puff pastry into a log, encasing the filling. Gently press down to seal the edges and repeat with remaining pastry and filling.
Cut each log into 8 pieces, about 1 inch wide, for a total of 32 pieces. Place mini rolls on the prepared baking sheet with the seam side down. Brush egg wash on top and bake for 20-25 minutes or until golden brown. Remove from oven and let cool slightly. Serve warm with apple mustard sauce.