1. Fold a kitchen towel into quarters and set it inside a large, wide bowl. Use one hand to hold an ear of corn in place upright atop the kitchen towel—it helps to pinch the ear at the top. With your other hand, use a chef’s knife to cut off two to three rows of kernels at a time by sliding the knife down the cob. Get as close to the cob as you can, and resist the temptation to cut off more rows at once—that’ll leave behind lots of precious corn. Save the cobs.
2. In a soup pot, quickly make a corn cob stock: cover the cobs with 9 cups water and bring to a boil. Reduce heat and simmer for 10 minutes, then remove the cobs. Set stock aside.
3. Return the pot to the stove and heat over medium heat. Add the butter. Once it has melted, add the onions and reduce heat to medium-low. Cook, stirring occasionally, until the onions are translucent, or blond, about 20 minutes. If you notice the onions starting to brown, add a splash of water and keep an eye on things, stirring frequently, to prevent further browning.
4. As soon as the onions are tender, add the corn. Increase the heat to high and sauté just until the corn turns a brighter shade of yellow, 3 to 4 minutes. Add just enough stock to cover everything, and crank up the heat to high. Save the rest of the stock in case you need to thin out the soup later. Season with salt, taste, and adjust. Bring to a boil, then simmer for 15 minutes.
5. If you have an immersion blender, use it to carefully blend the soup until it is puréed. If you don’t have one, work carefully and quickly to purée it in batches in a blender or food processor. For a very silky texture, strain the soup one last time through a fine-mesh sieve. (I’ve yet to do this.)
6. Taste the soup for salt, sweetness, and acid balance. If the soup is very flatly sweet, a tiny bit of white wine vinegar or lime juice ca help balance it out. (I add a fair amount of vinegar to balance out the sweetness — 2 to 3 teaspoons, maybe more. Remember, the acidity of the herb salsa, will balance the flavor as well.)
7. Serve hot or chilled with salsa spooned over top. Bread on the side is never a bad idea.

To make the salsa:

1. In a small bowl, combine the shallot and lime juice and let sit for 15 minutes to macerate.
2. In a separate small bowl (see notes above), combine the cilantro, jalapeño, scallions, oil, and a generous pinch of salt.
3. Just before serving, use a slotted spoon to add the shallot (but not the lime juice, yet) to the herb oil. Stir, taste, and add lime juice as needed. Taste and adjust salt.
4. Cover and refrigerate for up to 3 days.