In a large (10-inch) nonstick skillet set over medium heat, dry-roast the onion slices, turning them from time to time, until they are charred in spots and translucent, 4 or 5 minutes. (If you don’t have a nonstick pan, lay a piece of foil in your skillet to make clean up much easier.) Roughly chop the onion and scoop it into a food processor or blender jar and add the tomatoes, juices, hot sauce (or habanero) and pumpkin seeds. Blend until nearly smooth—this may take several minutes and will likely require you to stop the machine and scrape down the sides several times. The mixture should be a thick, dip-like consistency; if yours is thicker, blend in some water. Season with salt, usually a scant teaspoon, depending on the saltiness of your pumpkin seeds. Add the salt along with the cilantro to the food processor, pulsing several times to evenly distribute everything. Scrape the sikil pak into a serving dish and serve with chips or vegetables slices, or use it as a spread on a sandwich.