Melt butter over medium/medium high heat in Dutch oven or large pot. Add flour and whisk to combine.
Cook until roux has started to turn a light blond color, about 6 - 8 minutes.
Add tomato paste, onions, carrots, celery and garlic. Cook for another 6 - 8 minutes, until vegetables begin to soften.
Add stock, salt and pepper. Stir to combine.
Add potatoes and simmer, covered, for about 15 minutes, until potatoes are cooked through.
When vegetables are soft and fully cooked, remove from heat. Add cream and langostino tails.
Stir to combine and serve.