Place a large deep pan or dutch oven over medium-high heat. Add 2 Tbsp oil and once oil is very hot, add thinly sliced beef strips in a single layer, cooking 1 minute per side without stirring. Cook until just browned and no longer red. Sear beef in 2 batches (if you need to) so you don't overcrowd the pan. Remove beef to a plate and cover to keep warm.
Add 2 Tbsp butter, chopped spring onions and sliced mushrooms. Sautee 6-8 minutes or until liquid has evaporated and onions and mushrooms are soft and lightly browned.
Add 2 Tbsp flour and sautee for a minute stirring constantly.
Add half the broth while stirring. Once incorporated, add remaining broth.
Stir, then add sour cream and mustard. Stir until incorporated (don't worry if it looks split, sour cream will "melt" as it heats).
Bring to simmer ,then reduce heat to medium low. Once it thickens to the consistency of pouring cream (3 - 5 minutes), adjust salt and pepper to taste.
Add beef back in (including plate juices). Simmer for 1 minute, then remove from stove.
Serve over pasta, mashed potatoes or rice.