Heat 1 tablespoon olive oil in a large skillet over medium high heat. Add chicken and season with 1/4 teaspoon salt and 1/4 teaspoon pepper. Cook just until cooked through. Remove to a plate.: The pan will hiss as the oil meets heat, and you should hear a steady sizzle when the chicken hits the surface, indicating the Maillard reaction is happening. You want small golden browning rather than gray steamed meat, so do not overcrowd the pan. The aroma will shift from raw to toasty and savory as proteins brown, which signals proper searing. A common mistake is moving pieces too soon; let them form a golden crust so they retain juices. If the chicken looks pale, increase heat slightly for a minute, then lower it to finish. Transfer to a warm plate to rest, which locks in juices and prevents overcooking when it returns to the sauce.
To the now empty skillet, melt 2 tablespoons butter with 2 tablespoons olive oil over medium high heat. Add onions and cook 2 minutes then add the mushrooms and cook until they turn golden, about 6 minutes.: You will smell the sweetness of softened onion quickly, then the air will fill with rich, savory mushroom aroma as they release moisture and then brown. Listen for a gentle sputter that becomes quieter as the water evaporates and the mushrooms begin to caramelize. This color change to golden is crucial, because those browned bits build the base flavor for the sauce. Avoid stirring constantly; give the mushrooms space to brown. A typical error is crowding the pan which causes steaming instead of browning, so if the pan is watery, raise the heat briefly to evaporate liquid.
Add flour and garlic and cook, while stirring for 1 minute.: The flour will toast slightly, losing its raw edge and developing a faint nutty scent, which prevents a floury aftertaste in the sauce. The minced garlic will bloom, releasing fragrant oils and a warm, aromatic quality. Stir continuously so the flour cooks evenly and does not clump. You may notice the mixture darken a touch, which is fine, but do not let the garlic brown or burn, because a bitter note will carry through the sauce. If the pan seems too dry, add a splash of broth, whisking to incorporate.
Turn heat to low and slowly stir in beef broth followed by all of the seasonings and 1/4 teaspoon additional pepper.: As the beef broth hits the roux, you will hear a soft simmer and see the mixture smooth out. Adding the liquid slowly prevents lumps and allows the flour to hydrate evenly, creating a silky texture. The blend of bouillon, Worcestershire sauce , herbs, and spices will immediately enhance the aroma and deepen the sauce color. Keep the heat low to avoid a rapid boil that can separate dairy later. A frequent mistake is pouring the broth too fast, creating lumps, so pour in a thin stream while whisking or stirring thoroughly.
Bring to a boil then reduce to a gentle simmer until thickened (but not too thick), about 5 minutes.: You will see small bubbles across the surface as it reaches a boil, then calming into a gentle simmer, at which point the sauce will visibly thicken and coat the back of a spoon. The smell will concentrate, with savory notes becoming more forward. This reduction is where flavors concentrate, so tasting and adjusting seasoning is key. Avoid over reducing, which can make the sauce gluey and overly salty; if it becomes too thick, loosen with a splash of broth or water.
Reduce to low and whisk in cream cheese until melted (it will take about 5 minutes if you really want it all melted – it will still taste good if it's spotty). Add sour cream and stir until combined. Add chicken back to pot and warm through, about 1 minute.: As you whisk in softened cream cheese , the sauce will become glossy and velvety, releasing a rich aromatic cream note. The whisking motion breaks down small pieces and blends them into the sauce, smoothing texture. You will want to keep the heat low to avoid curdling the sour cream , and when it blends in the sauce should have a gentle tang that balances richness. Return the chicken to warm through briefly, maintaining tenderness. A common error is overheating after adding dairy, which causes separation, so keep the flame low and stir gently until combined.
Serve over egg noodles/pasta of choice or mashed potatoes, rice, etc. Garnish with fresh parsley if desired.: The final plate will present a glossy, creamy sauce ladled over egg noodles , where the sauce clings to each strand, offering a comforting mouthfeel. You will notice a final aroma of herbs and a gentle tang from the sour cream. If you like a fresh contrast, sprinkle chopped parsley for brightness. Avoid leaving the dish sitting in a hot pan, which can tighten the sauce and dry the chicken ; instead, serve promptly so textures stay ideal.