Prep the beef: Pat the beef dry. Slice thinly against the grain.
Season with salt, pepper, and a pinch of smoked paprika if you like.
Spiralize the zucchini: Make zoodles with a spiralizer or buy pre-spiralized. Spread on paper towels and sprinkle lightly with salt. Let sit to draw out moisture.
Sear the beef: Heat a large skillet over medium-high.
Add oil. Sear beef in two batches, about 1–2 minutes per side, just until browned. Remove to a plate.
Sauté mushrooms and onions: Add butter to the skillet.
Toss in onions and mushrooms with a pinch of salt. Cook 6–8 minutes, stirring, until browned and the liquid evaporates.
Add garlic and deglaze: Stir in garlic for 30 seconds. Pour in the beef broth, scraping up browned bits.
Reduce heat to medium.
Build the sauce: Stir in Worcestershire and Dijon. Simmer 3–4 minutes to reduce slightly.
Thicken (if needed): Sprinkle in xanthan gum and whisk, or stir in a slurry of arrowroot with a splash of cold water. Simmer 1 minute until lightly thickened.
Add sour cream: Lower heat.
Whisk in sour cream until smooth. Do not boil to prevent curdling. Adjust salt and pepper.
A small squeeze of lemon brightens the sauce.
Return the beef: Add the seared beef and any juices back to the pan. Warm gently 1–2 minutes until just cooked through and tender.
Cook the zoodles: Heat a separate skillet over medium-high with a little oil. Pat zoodles dry and cook 1–2 minutes, tossing, just to soften.
Don’t overcook.
Serve: Plate zoodles and spoon stroganoff over the top. Finish with chopped parsley and fresh pepper.