Cube the beef and remove any excess fat. Season well with salt and pepper. Brown in a large pot with the oil and 1 Tbsp. butter. Remove and set aside.
Brown onions in the same pan for 4-5 min. Return the steak and add stout, beef stock, bouquet garni and top with the sliced potatoes. Cover the pot with a tight fitting lid and let simmer for 1 hour.
Add the mushrooms without stirring and cook for another 30 min.
Remove the meat and potato mixture onto a large serving platter.
Mix the remaining 1 Tbsp. of butter with the flour and whisk into the liquid to make a gravy. Whisk in the mustard and let cook until thickened for about 3 min. Taste and adjust seasoning.
Pour over meat and potatoes and serve garnished with thyme sprigs.