Dust the meat and mushrooms with flour and season with salt and pepper.
In a large oven proof pan, sear the beef in olive oil until golden brown. Be careful not to burn the flour. Set beef aside.
In the same pan, brown the button mushrooms in the olive oil for about 5 minutes, then add the chorizo, onion, leek, celery and carrots and cook until soft. Add the passata, chicken stock, Worcestershire sauce and thyme. Mix well and bring to a simmer for 8 minutes.
Meanwhile, boil the sweet potato in a separate pot, until soft. Strain and return to the same pot on a low heat. When back on the stove top, mash the sweet potato with butter and cream using a potato masher or a whisk.
To finish the pie, stir the beef through the saucy mixture. Cover with the sweet potato mash using a spatula. Scrape the surface with a fork to create small peaks.
Place in the oven, on grill, for 10 min till caramelised.