Place the egg, milk, flour, salt, and spinach in a blender and pulse until the batter is smooth. Place a 6-inch, well-seasoned or nonstick crêpe pan over medium heat. Dip the cut side of a potato half in the melted butter and use it to lightly grease the pan. Tilt the hot pan toward you. Ladle a scant ¼ cup of the batter into the front of the pan, then slowly tilt the pan backward to evenly coat the bottom with batter. When bubbles appear on the top, carefully turn the crêpe with a spatula and cook for 20 to 30 seconds on the other side. Remove the crêpe to a plate and repeat with the remaining batter. If you are planning to serve the crêpes immediately, stack them and cover the plate loosely with aluminum foil to keep them warm. If making the crêpes in advance, let the crêpes cool, then stack them with sheets of baking parchment or waxed paper between the crêpes. Wrap the stack in aluminum foil and refrigerate for up to 2 days or freeze for up to 1 month. When ready to use, either separate as many crêpes as you need and let them thaw or wrap still frozen crêpes individually in foil and warm them in a 300°F. oven for about 10 minutes.