Cut the pork fillet into medallions of the same size. Season with salt and pepper, fry on both sides for approx. 2 minutes and set aside wrapped in aluminum foil.
Sauté the onions and the clove of garlic in the same pan until translucent, and add the diced tomato pieces. Let it sweat as well, then add the tomatoes and herbs. Season to taste with salt and pepper and let everything boil down for a few minutes. When the sauce has the desired consistency, insert the medallions and cover them over a very low heat for about 15 minutes.
Meanwhile cook the tagliatelle al dente.
Season the sauce again to taste and serve everything.