In a small saucepan, cover potatoes with cold water and boil until tender, about 12 minutes. Drain well and transfer to a medium bowl. Mash with a potato masher until fairly smooth. Add onion, goat cheese, butter and olive oil and stir until combined and creamy. Add thyme, zest, salt and pepper and stir to combine.

Roll out puff pastry on a lightly floured surface to about 1/8” thick. Cut into 24 squares. Place a heaping teaspoon full of potato filling in the center of each square. Brush 2 of the 4 edges of each square with egg wash and fold edges over to create a triangle. Slightly press down filling and press edges tightly to seal. Press edges with the tip of a fork to seal and transfer triangles to a sheet pan. Chill 20 minutes or until firm.

Preheat oven to 425℉ . Brush the tops of triangles with remaining egg wash and use a small knife to cut a ¼” slit in the top of each to let steam escape while baking. Bake 10 minutes. Reduce oven to 350℉  and bake 10 minutes more or until golden brown. Serve warm.