Defrost the fish fillet. Rinse fish, dab dry, cut into cubes. Mix 3 tablespoons of oil with the crushed garlic and let the fish cubes soak in it for a few minutes.
In the meantime, cook gnocchi in salted water according to package instructions, drain (reserving some of the cooking water). Peel and dice onion. Wash and halve tomatoes.
Heat 1 tablespoon oil, sauté onion. Add cream cheese and pasta water, mix. Heat remaining oil in a frying pan, fry salmon cubes in it over medium heat for 3 minutes, turning from time to time, season with salt and pepper. Mix gnocchi with tomatoes and cheese sauce, add salmon. Serve sprinkled with chives, if desired.