
Start by adding the beef steak to a pot with the mustard, submerge the steak and cover the pot.
Place it in a preheated oven at 180° C (350° F) and bake for 30 minutes.
In a large pot, add the potatoes cut into small cubes and enough water to cover everything.
Turn on the heat to medium and let them cook for 15 minutes.
After this time has passed, drain all the water and transfer them to a large bowl.
Add the butter, the salt and mash everything very well until it becomes a puree.
Next, add the grated Parmesan cheese, mix once more to incorporate and set aside.
After the steak has spent the indicated time in the oven, remove it from the pot.
Season with the salt, the black pepper, the chimichurri, flip it and repeat the same seasoning on the other side.
In a large skillet over medium to low heat, add the butter, the peeled garlic cloves, the sprig of rosemary and stir everything very well until the butter is completely melted.
Add the seasoned beef steak and fry it until both sides are golden brown.
Finally, cut the beef steak into strips.
Serve.