Put the potatoes and garlic in a saucepan, cover with water and add 1 teaspoon salt. Bring to a boil, lower to a simmer and simmer until the potatoes are very soft and tender, 18 to 20 minutes.
Preheat the oven to 425 degrees F. Line a baking sheet with parchment paper.
Drain the potatoes and garlic, transfer to a bowl and mash with a potato masher until almost smooth but with a slight chunk to them. Add the spinach, Cheddar, cream cheese, Parmesan, sour cream, 1 teaspoon salt and 1/4 teaspoon pepper. Mix until well combined.
Put the sheet of puff pastry on a lightly floured surface and roll it out so it's about 11 inches square. Brush off any excess flour. Cut the sheet into 9 pieces and add 1 1/2 tablespoons of the mashed potato mixture to the center of each. Brush with the egg wash and bring the corners up to create purses. Place the purses seam-side up on the prepared baking sheet.
Brush the tops with some more egg wash and sprinkle with Parmesan. Bake until golden, about 20 minutes.