Put chicken, onion, hot sauce and butter in a slow cooker. Cover and cook on low for 8 hours. Remove chicken and shred into small pieces. Scoop out about ½ cup of the liquid from the slow cooker and stir into the shredded chicken. Taste and add a little more hot sauce if you like things spicier.Preheat oven to 400. Place the tortilla chips on an oven-safe platter or a cookie sheet. Get them as close to a single later as you can.Spread 1 cup of the cheese over the chips. Spread the shredded chicken over the cheese, and sprinkle the rest of the cheese on top. Bake until the cheese is just melted, about 10 minutes.Drizzle the blue cheese dressing over the nachos. Sprinkle the tomatoes and scallions on top and serve at once.