Preheat oven to 350°. Prepare an ice bath: Fill a bowl with cold water and add ice cubes. Roast the tomatillos 20–30 minutes, or until lightly charred. Strip the root beer leaf of its center stalk and blanch the leaf in boiling water briefly (about 15 seconds). Remove with a slotted spoon and immediately put in the prepared ice bath. This helps to preserve the chlorophyll (the green pigment in the leaves). Drain the leaf and put in a blender, along with the roasted tomatillos, Serrano (add more or less depending on the heat you like), cilantro, shallot and lime juice, and pulse until it emulsifies.
Serve this salsa as is with blue corn chips or as a side with roast chicken, grilled seafood, pork chops, steak, or with sweet potato fries, for instance.