One of my hobbies is buying too many herbs and then watching watching them wilt, dehydrate or mush out in the fridge, depending on how I’ve (incorrectly) stored them. Would placing my cilantro in a jar of water extend its life? Yes. Am I going to do that? You know I am not. (There simply isn’t a ton of room for tall things in my tiny, dumb fridge.)
But even when herbs look wilted and pathetic—and I’ve seen some very wilted and pathetic herbs in my time—they still have a good bit of flavor trapped inside their cellular walls, and you can free that flavor by blending them into some mayonnaise, even if they’ve turned to mush. If you have a bunch of tender herbs (like cilantro, parsley or sage) you can toss the whole shebang in there, stems and all. If your herbs are tragic-looking but woody (like rosemary and thyme), you will need to remove them from their stems first. You can also chuck shriveled, pale scallions in there—including the roots—if you have any languishing in your produce drawer.
Aim for a ratio of one cup of mayonnaise for every cup of roughly chopped herbs and alliums. Wash the plant parts by swishing them around in a bowl of water, pat them dry with paper towels, and blend them with the mayo using a food processor, immersion blender, or “regular” blender until you have a green, super herby, fresh but still very creamy sauce. It’s very good as-is—spread it on a sandwich!—but you can also thin it out with a little red wine vinegar to make a salad dressing (add a pinch of sugar as well), or mix it into sour cream to make a more substantial chip dip. It’s also friendly to potatoes of all kinds: it’s delicious drizzled on roasted potatoes or whipped it into a mash, and it makes a superb french fry dipping sauce.