Peel onions and carrots and cut into fine cubes. Heat the fat in a pan and sauté the onions and carrots. Add tomato paste and deglaze with stock. Cook over medium heat for 10 minutes.
Season to taste with salt and pepper. Cook the pasta in plenty of boiling salted water for 10-12 minutes. Drain tuna on a sieve and cut into small pieces. Drain the capers. Add the fish and capers to the sauce and heat briefly.
Wash parsley, dab dry and cut into strips. Drain the pasta and serve with the tuna sauce. Serve sprinkled with parsley.