Marinate chicken in bbq sauce for 2 hours in the refrigerator. Remove from marinade and grill each thigh for 6 to 7 minutes on each side or until cooked through.
Mix all ingredients for dressing, and season with salt and pepper.
Arrange lettuce on 4 plates, top each plate with a chicken thigh. Garnish with blue cheese, avocado, onion and tomato. Drizzle with the buttermilk dressing.
In a large saucepan over medium heat, melt the butter and sweat the onion and garlic until translucent, about 3 minutes. Add the tomatoes and simmer 15 minutes. Add the remaining ingredients and simmer 20 minutes.
Puree the mixture in a food processor, pour into a bowl, and let cool at room temperature. May be refrigerated up to 1 week or frozen.