Trim your steak from any fat, and slice it thinly into 1/4 inch against the grain. Cut any long strips in half. Season with salt.: The first sensory cue is the clean sound of the knife as it slices through the chilled steak , which should feel firm and slightly cool. Slicing against the grain shortens muscle fibers, so the meat feels tender when chewed. If the slices feel stringy you likely cut with the grain, and the texture will be tougher after cooking. Take your time and use a sharp knife, pressing evenly so slices are uniform and cook at the same rate.
In a large skillet over medium-high heat, heat olive oil and melt the butter. Sear the beef strips in a single layer in batches, 30 seconds per side, then remove onto a plate and set aside.: You want the pan hot enough that the olive oil shimmers and the butter foams, producing an immediate sizzle when the steak hits the surface. The smell of caramelizing meat and browned butter is your cue that the fond is forming. Searing in a single layer ensures even contact and a golden crust, while removing in batches prevents steaming. A common misstep is overcrowding, which causes grey, steamed meat instead of browned slices. Let the pan reheat between batches for consistent color.
In the same skillet, melt butter and cook onion until soft and translucent, about 3 minutes.: You will notice the aroma shift to sweet as the onion softens, and the pieces turn glossy and pale. This gentle cooking releases sugars that balance the savory elements of the dish. Stir occasionally so the edges do not brown too quickly, unless you prefer deeper caramel notes. If the heat is too high the onion will brown unevenly and add a bitter edge, so keep the skillet at medium.
Add mushrooms, and cook for 3 more minutes or until they start to soften. Add garlic, and saute for 30 seconds or until fragrant.: As the mushrooms hit the pan you should hear a lively sizzle and then a slower release of moisture, which concentrates their earthy aroma. They will shrink and take on browned edges, signaling flavor development. Add the minced garlic later to avoid burning, and watch for its brief fragrant moment; overcooked garlic becomes bitter. If mushrooms steam rather than brown, they were crowded in the pan or the heat was too low.
Sprinkle flour all over the mushrooms, stir, and cook for 30 seconds.: The dry smell of the flour will quickly change as it heats and becomes nutty, which is what you want before adding liquid. This toasting step cooks out raw starch and helps the sauce thicken evenly. Stir thoroughly to coat the mushrooms and aromatics so no dry pockets remain. If you skip the toast the sauce can taste pasty or have raw flour notes.
Pour the beef broth into the pan, deglaze it, and scrape off any bits stuck to the bottom of the pan. Stir, lower the heat to medium, and allow the mixture to simmer and thicken for 5 minutes.: When the beef broth hits the warm pan you will hear a lively hiss and watch the browned bits lift, turning the liquid a rich color as you scrape. Reduce to a gentle simmer so the flour hydrates and the mixture thickens into a glossy sauce. Smell and taste as it reduces; the aroma should be concentrated and meaty. If it bubbles too hard the sauce can break or reduce too far, becoming salty or pasty.
Add Worcestershire sauce, Dijon mustard, and sour cream to the sauce. And stir until well incorporated.: The moment you stir in the Worcestershire sauce and Dijon mustard the sauce gains a layered tang, and folding in the sour cream brings silkiness. Do this off or on very low heat to avoid curdling, and stir until the texture is uniformly glossy. If the sauce begins to split, lower the heat and whisk gently while adding a splash of warm beef broth to stabilize it.
Add sauteed beef strips to the sauce with any juices left on the plate, stir to combine, and simmer for 1 minute. Remove from heat and serve over egg noodles, garnish with chopped fresh parsley or chives.: When you return the beef to the pan, listen for the soft reunion sizzle and watch the sauce cling to the meat, indicating it has properly bonded. Simmering briefly allows flavors to meld without overcooking the thin slices, which should remain tender and slightly pink if you prefer. Overcooking here is the most common error, leading to dry strips, so keep the final simmer to about the recommended time. Serve immediately so the sauce stays glossy and the textures remain distinct.