Prep the beef. Pat the beef dry and slice it thinly across the grain.
Season lightly with 1/4 tsp salt and a pinch of pepper. This helps sear quickly and stay tender.
Sear in batches. Heat 1/2 tbsp olive oil in a large skillet over medium-high. Add half the beef in a single layer and cook 1–2 minutes per side until just browned.
Remove to a plate and repeat with the remaining beef. Do not overcook.
Soften the aromatics. Lower heat to medium. Add the remaining 1/2 tbsp olive oil.
Add onion and a pinch of salt. Cook 4–5 minutes until translucent.
Cook the mushrooms. Add sliced mushrooms. Let them sit for a minute to brown, then stir occasionally for 5–6 minutes until they release liquid and turn golden.
Add garlic and flavor base. Stir in garlic, tomato paste, Dijon, Worcestershire, and paprika.
Cook 1 minute, stirring, until fragrant and the tomato paste darkens slightly.
Build the sauce. Whisk flour into the pan for 30 seconds (or stir cornstarch into a few tablespoons of cold broth, then add). Gradually pour in the beef broth while stirring, scraping up browned bits. Bring to a gentle simmer.
Simmer and thicken. Let the sauce bubble for 3–5 minutes until lightly thickened.
Taste and adjust salt and pepper.
Return the beef. Add the seared beef and any juices back to the skillet. Cook 1–2 minutes to warm through. Remove from heat.
Finish with yogurt. Let the pan cool for 1–2 minutes to avoid curdling.
Stir in Greek yogurt until smooth. Add a squeeze of lemon if you like a brighter sauce. Sprinkle with parsley.
Serve smart. Spoon over whole-wheat noodles, zucchini noodles, or cauliflower rice.
Portion 3–4 oz cooked beef per serving for a balanced meal.