In a large Dutch Oven, heat 1 tbs oil over medium-high heat. Season the beef with salt & pepper and sear on all sides, 2 to 4 minutes. Remove from pan & set aside. Cover with foil.
In the now empty pan, add the butter & swirl to coat. Add the onion & cook 5 minutes, until soft. Add the mushrooms & cook until water has evaporated, & they are browned.
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.