In a large Dutch Oven, heat 1 tbs oil over medium-high heat.
Season the beef with salt and pepper and sear on all sides, 2 to 4 minutes.
Remove from pan and set aside. Cover with foil.
In the now empty pan, add the butter and swirl to coat.
Add the onion and cook 5 minutes, until soft.
Add the mushrooms and cook until water has evaporated, and they are browned.
Add the garlic, and cook 30 seconds until fragrant.
Sprinkle with the flour and cook for 2 minutes, stirring often.
Whisk in the stock, 1 cup at a time making sure no clumps form.
Add the thyme and bring to a boil, stirring every now and then.
Add the potatoes and reduce heat to maintain a simmer.
Cook for 15-20 minutes, until potatoes are tender.
Stir in the cream, mustard, and steak sauce and simmer 5 minutes longer.
Divide the steak between bowls and ladle soup over top.