Heat the fat in a pan on medium heat. Briefly fry the sliced beef all around, season and put aside.
Reduce the heat and sweat the shallot, spring onion and garlic, add the mustard and stir well. Pour in the stock, put the meat back in the pan, put the lid on and let cook for about 30-40 minutes.
For the last 15 minutes, cook the spaghetti in salted water until firm to the bite, pour off and arrange on a plate.
Pour the shredded meat over the pasta, braise the sliced tomato briefly in the remaining heat of the pan and drape it on the plate.