With a hat-tip to Bon Appetit

butter
2 medium leeks, white and light-green parts only, coarsely chopped (about 1 1/2 cups); can substitute yellow onions
5 ears shucked corn, kernels cut from cobs, cobs reserved
1 cup coarsely chopped peeled potato (about 1 medium)
4 cups good-quality vegetable stock
Kosher salt and freshly ground black pepper
1 tablespoon fresh lemon juice (optional)
1/4 cup crème fraîche or sour cream
1 tablespoon finely chopped fresh chives

Cook leeks in oil until soft. Saving some corn kernels for toppings, add most of the kernels, cobs, potatoes and stock. Bring soup to a boil at high. Reduce to simmer, cover with lid slightly ajar, and cook 30-60 minutes, until the vegetables are soft. The longer you cook, the more flavor will develop.

Remove cobs and puree the rest of the soup. Strain (optional). Stir in lemon juice, and season with salt and pepper. Top with crème fraîche, reserved kernels, and chives (optional). Can be served hot, cold, or warm.