Heat the oil in a 4-quart pot until shimmering. Add the onion and saute until lightly golden, 1 to 2 minutes. Add the corn, potatoes, sage and chile pepper flakes; saute for 1 minute. Season with salt. Stir in the broth and bring to a boil. Reduce the heat to medium low and simmer until the potatoes are tender when pierced with a fork, about 5 minutes.
Carefully pour half the mixture into a blender and puree until smooth-remember the soup is hot, so be careful. Stir the pureed mixture back into the pot with the remaining soup. Taste the soup and add more salt, if desired. Ladle into bowls, garnish with a few chile flakes and serve hot.
Using a sharp knife, cut the kernels off the cob (see Cook's Note); reserve the kernels for a later use.
Using the side of a spoon or a butter knife, scrape down the sides of the cobs into a deep stockpot to release the "milk" and loosen any bits of corn. Add 8 cups water to the pot, along with the salt, peppercorns, garlic, bay leaf and onion. Bring to a boil over medium-high heat. Reduce the heat to medium low and let the broth cook for 20 minutes. Let cool slightly.
Pour the broth through a fine-mesh sieve to strain out the vegetables and bits of corn (a pot lined with cheesecloth works well too). The broth is now ready to use as a base for soups, stews and sauces. If not using immediately, transfer to glass jars and let cool completely. Store tightly covered in the fridge for up to 1week, or in the freezer for up to 2 months.
Easy Upgrade: Crispy fried sage leaves add elegance to this soup with little effort. Heat 2 to 3 tablespoons of grapeseed oil in an 8-inch nonstick skillet. Add the leaves a few at a time. They'll shrink and crisp up in a few seconds. Use a fork to lift them out of the pan, and let them drain on a paper-towel-lined plate for 1 minute. Use immediately to garnish the soup. Technique Tip: Break the ears in half before cutting off the kernels. The smaller size and flat edge where it's broken allows for a sturdier grip when holding the cob on a cutting board.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.