Season the steak all over with 1.5 Tsp. salt
In a large skillet, heat 1 Tbsp. of the olive oil over medium-heat.
Working in small batches, add the steak and cook, undisturbed, until browned, 1-2 minutes per side.
Transfer to a plate. Repeat with the remaining steak, adding more olive oil as needed.
Reduce the heat to medium. Add the butter to the same skillet.
Once the butter has melted, add the onion and mushrooms and cook until the onion softens and the mushrooms are starting to brown.
Add the garlic and cook, stirring, until fragrant.
Stir in the flour and cook, stirring for about 1 minute. Slosly our in the beef broth and stir until fully incorporated.
Add the thyme. 1/2 Tsp. salt & pepper and simmer, stirring often, until the mixture is starting to thicken.
Stir in the Worcestershire sauce, sour cream, and Dijon mustard, until the sauce is combined.
Bring the sauce to a gentle simmer and cook until the sauce is thick enough to coat the back of a wooden spoon.
Chop the meat, and return to the skillet and toss until fully coated.
Cook pasts or whatever you diced to serve it with.
Serve over pasts, egg noodles, rice, or mashed potatoes.