Slice leeks thinly, width-wise. Over medium heat, melt the butter and olive oil in a large soup pot. Saute the leeks for 10-15 minutes.
Add the blue cheese to the pot, stirring until the cheese is melted. Then add the flour, stirring constantly until absorbed. Mix in the mustard and black pepper.
Add the chicken broth and turn heat to high. Stir until soup comes to a boil. Reduce heat, cover and simmer for 15 minutes.
Serve. Garnish with additional blue cheese and the diced scallions.