Bring a large pot of salted water to a boil. Add the pasta and cook for 8 to 10 minutes or until al dente or according to package directions. In a sauce pot heat butter over medium heat. Add flour and cook until bubbling, about 20 seconds. Add milk a little bit at a time and whisk until smooth. Bring sauce to a boil and simmer 3 minutes. Whisk in tomato paste. Add the fish strips and cook, uncovered, until fish is just cooked through, about 2 minutes. Add mint, and season to taste with salt and pepper. Drain pasta, divide it among 4 shallow bowls and ladle fish sauce over each portion.