Bring a large pot of generously salted water to a boil. Add the spinach. As soon as it's wilted and tender, drain and run under cold water. When the spinach is cool enough to handle, squeeze out any excess water. If you have used whole spinach with long stems, roughly chop it; you should have 1 1/2 cups of chopped spinach.
Slice half of the onion into thin disks, and break the disks into rings with your hands. Mix 1 tablespoon of the flour with a bit of salt and pepper, and toss the onion rings in the flour.
Heat 1 inch of grapeseed oil to 350 degrees F in a large, deep saute pan or Dutch oven. Add a handful of the onion rings to the oil, making sure they have room to move. Fry in the oil until golden brown, 3 to 4 minutes. Transfer to a paper-towel-lined plate to drain. (If desired, keep the fried onions warm in a 200 degrees F oven.)
Finely mince the fennel and the remaining onion half. Heat a medium saute pan over medium heat and glaze it with the olive oil. Add the fennel, onion, garlic and a few pinches of salt, and cook until the onion is translucent; you don't want it to brown. Add the remaining 1 1/2 tablespoons flour and stir. When the flour has gathered up all the onion mixture, add the Herbsaint and bring to a boil. Let it reduce by half before stirring in the milk to make a bechamel. Season to taste with salt and pepper. Cook at a low simmer until the sauce thickens to the point where it coats the back of a spoon, and where drawing your finger down the spoon leaves a clear line.
Pour half of the bechamel into a blender along with 1/2 cup of the spinach. Blend until completely smooth. Add the remaining spinach to the pan with the remaining bechamel, then add the spinach puree. Heat, stirring, and season with salt and pepper. Finish by stirring in the butter, nutmeg and Parmesan. Transfer the creamed spinach to a serving dish, and top with the fried onions.
Herbsaint is a liqueur made in New Orleans. Think of it as the American version of Pernod. It has a minty-anise flavor.