In a large Dutch oven or soup pot melt the butter over medium heat. Add the onions, celery, carrots, garlic and stir. Season generously with salt and pepper and cook for about 5 to 7 minutes or until the onion and carrots soften.
Meanwhile, season the chicken thighs with the cajun seasoning and toss together with the buffalo wing sauce, making sure each thigh is covered in cajun spice and sauce. Grill the chicken on a grill pan for about 4 minutes per side, just until you get some grill marks. The chicken doesn't have to be cooked through completely, as it will cook some more in the soup. Remove from grill pan and shred with 2 forks.
Sprinkle the flour over the vegetables and stir. Add tomato paste, chicken broth, wing sauce, sour cream, blue cheese, blue cheese dressing and stir well, so that the tomato paste dissolves. I use a whisk to make sure it's stirred well. Bring to a boil.
Add the shredded chicken to the pot and turn the heat down to a simmer. Simmer for 30 minutes.
To serve, garnish with parsley, top with blue cheese and green onions.
