Turn on a gas grill to medium or light a charcoal fire, letting the coals burn until they are covered with a gray ash and are medium-hot. If you have wide grates, place a perforated grill pan over the grates. Lay the tomatillos, onion and pineapple slices directly over the fire and grill on all sides until everything is splotchy black in places and soft, about 15 minutes depending on the heat of your fire. Cool all the roasted ingredients, then cut the pineapple and onion into ¼-inch pieces and finely chop the tomatillos. Combine in a bowl with the jicama and chiles. Taste and season with salt, usually about a teaspoon, and a little sugar, about ¼ teaspoon, to balance the tanginess if you like.