For the sauce: In medium saucepan over medium heat, add the butter, garlic and jalapeno. Saute for 3 minutes, and then sprinkle in the flour. Cook, stirring frequently, until you create a blond roux, about 5 minutes. Add the milk, cheese and mustard and use a whisk to stir all together. Season with salt and pepper and let cook for 5 minutes on low.
For the nachos: Shred the chicken and place into a medium bowl. Pour in the barbecue sauce and mix well to combine. Arrange the tortilla chips on a platter and spread the chicken over the chips. Pour the warm cheese sauce over the top and sprinkle with the jalapenos and cilantro. Garnish with dollops of the sour cream.