SWEET POTATO PUFF PASTRY GALETTE

Name: SWEET POTATO PUFF PASTRY GALETTE
Cuisine: European
Ingredients: 4 sweet potatoes, about 2 pounds total; flour, for dusting the work surface; 1 frozen puff pastry sheet, thawed; 1 egg, beaten; ⅔ cup sour cream; ½ cup finely chopped fresh baby spinach; 2 cloves garlic, minced; ½ cup crumbled feta cheese; 1 tablespoon olive oil; a big handful or two of chopped walnuts; salt and fresh black pepper