You might find them in your garden or yard (only harvest nettles from areas that aren’t sprayed with pesticides or other potentially harmful chemicals), at specialty markets, or in tangled piles at farmers markets.
With your hands covered with rubber gloves or plastic bags, rinse the nettles clean in cool running water. Nettles can be quite gritty, so make sure they are fully and totally clean.
Peel and roughly chop the onion and potatoes.
In a large pot over medium-high heat, melt the butter. Add the onion and the salt and cook, stirring occasionally, until the onion is soft, about 3 minutes.
When the onions are soft, add the potatoes and the broth. Simmer until the potatoes are tender to the bite, 10 to 15 minutes.
Add the cleaned nettles and cook until they’re tender, about more 10 minutes. Stir in the pepper and nutmeg.
While the potatoes and/or nettles cook, in a small bowl, stir the crème fraîche and horseradish together. Set aside.
Use an immersion blender to thoroughly purée the soup or whirl the soup in a blender (in batches, if necessary). For a silky smooth texture, run that purée through a food mill (extra work and totally optional!). Adjust the salt and pepper to taste.
Serve hot, with the horseradish cream on the side for diners to add as they like.
Nettles are often called stinging nettles for a reason: they sting. Wear gloves (or plastic bags) on your hands when handling them when they’re raw—once they’re cooked, all the sting is gone.