Season the steaks with salt and pepper.
Heat oil in a frying pan and sear steaks for 3 minutes per side.
Add vinegar, half the brandy, and flambé carefully.
Add stock, garlic, and thyme, cook for 3 minutes, then rest the steaks.
Reduce the remaining liquid with peppercorns and brandy for 10 minutes.
Stir in double cream and simmer until thickened.
Whisk egg yolks, vinegar, and water over simmering water until fluffy.
Drizzle in melted butter while whisking to emulsify.
Stir in tarragon and a splash of olive oil.
Boil potatoes for 12 minutes until tender.
Mash potatoes with butter and cream until combined.
Fry mushrooms in butter until golden, adding garlic and thyme at the end.
Serve steaks with mash, mushrooms, and your choice of sauce.