Roast the poblano chili directly over a gas flame or 4 inches below a very hot electric broiler, turning regularly until blistered and blackened all over, about 5 minutes for an open flame, about 10 minutes for the broiler. Cool until handle-able, rub off the blackened skin, tear open, and pull out the stem and seed pod. Quickly rinse to remove any seeds or bits of skin. Roughly chop and scoop into a food processor, along with the serranos and cilantro. Pulse until uniformly chopped, then run the machine until you have a coarse puree.