Melt the butter in a medium pot over medium heat. Add the onion and sprinkle with salt and pepper. Cook until the onion is soft and translucent, 5 to 7 minutes. Add the garlic and cook for about a minute longer. Stir in the flour until combined and cook for 2 minutes to cook out the flour taste. Add the heavy cream and cook, stirring, until the sauce is thick and there are no lumps, about 5 minutes.
Add the spinach, breaking up any frozen chunks with the back of a wooden spoon. Stir to combine with the creamy mixture until uniformly incorporated. Taste and season with salt. Cook until the spinach is heated through and mixture begins to bubble, about 3 minutes. Add the Cheddar, lemon juice and hot sauce. Season with salt and pepper. Turn to the lowest setting.
Heat a large nonstick pan over medium-high heat and line a baking sheet with parchment paper. Make 2-tablespoon mounds of Parmesan in the pan, leaving 3 inches between each (work in batches if your pan isn't large enough). Allow the Parmesan to melt and bubble until it starts to brown around the edges. Use an offset spatula to carefully lift each cheese crisp to the lined baking sheet. Allow to cool for 2 minutes. Crumble or serve whole on top of the creamed spinach.