Beef cooked in a milk carton

Start by seasoning the piece of beef with coarse salt to taste and spread it well with your hands. 

Place the meat in the one-liter milk carton with one of the sides cut open and keep seasoning it. 

Season with more coarse salt, a pinch of dried rosemary and mix well. 

Close the carton, place it on top of a large sheet of aluminum foil and wrap it up. 

Transfer it to a large baking tray, take it to a preheated oven at 200° C (400° F) and cook for 40 minutes.

In a medium saucepan, add the potatoes cut into smaller pieces and add enough water to cover them. 

Add the salt, turn the heat on high and let them cook for 15 minutes.

After this time has passed, transfer the potatoes to a large bowl. 

Add the unsalted butter and mash everything until smooth. 

Next, add a pinch of salt, the grated parmesan cheese, the heavy cream and mix very well. 

Add parsley to taste and mix once again. 

Once the meat has cooked through, remove it from the carton and transfer it to a baking tray lined with non-stick paper.

Put it back in the oven and let it cook for another 20 minutes.

After this time has passed, remove it from the oven and cut it into strips.

Serve it with the mashed potatoes.