Set a large (10-inch) nonstick skillet (or a heavy skillet lined with foil) over medium heat and lay in the onion slices. When the onion is browned and softened on one side (about 4 minutes), turn the slices over and brown the other side (about 4 minutes more). Cool to room temperature, roughly chop them and scoop into a blender or food processor. Add the pumpkin seeds, tomatoes, juices, sesame seeds and hot sauce and process until completely smooth. Stir in the cilantro, then taste and season with salt, usually a scant teaspoon depending on the saltiness of the pumpkin seeds. Transfer to a serving bowl and serve with raw jicama, cucumber slices and tortilla chips.