Place the flour, milk, salt, and eggs in a blender. Blend until smooth. Stop to scrape the sides occasionally. Let rest 20 minutes.
If you just have one blender bowl, transfer the batter to another bowl, and let it rest. Wash the blender so it's ready for the Hollandaise Sauce.
Preheat a saute pan with a small amount of the oil over medium heat. Set a platter next to the stove.
Reserve ⅓ of the parsley for decoration, then add the onion (if using) and herbs to the rested batter, stir until they are mixed in.
Use a scoop or ladle to add ½ cup of batter to the hot pan. Swirl it around. Then let it sit. Use a spatula to check underneath. When it's browning, flip it over. (about a minute or so). Let cook for another 45 seconds to a minute, check that it's browned underneath. Slide the pancake out of the pan onto the waiting platter.
Repeat until you've used up all the batter. Occasionally you will want to add more oil to the pan... I find that you need to do that every other pancake.