Defat the sirloin, season with mustard and allow to rest in the refrigerator for 30 minutes.
Meanwhile, thinly julienne the onion and pit the cherries. Once the 30 minutes in the fridge have passed, sear the sirloin in a pot with olive oil, very hot, forming a brown crust on all surfaces. Then take out of the pot and set aside.
Put olive oil again in the same pot and fry the onions seasoned with salt and pepper. Once they start browning, add the pitted cherries, stir well and put the sirloin back in the pot. Add port wine, broth, and allow to cook over medium heat, covered, for 30 to 40 minutes.
Cook the potatoes separately, well washed, with skin and seasoned with salt in plenty of water. Once cooked, strain the potatoes and put them in an oven tray. Crush them with your hands until they break a little and add butter and some more salt and pepper. Put in the oven (400°F) for 30 to 40 minutes or until the edges are brown.
To serve, first cut the sirloin and then put 2 pieces of sirloin on top of a squashed potato, along with a few cherries and onions.