Season Beef: Season the beef (2 pounds) with 1 1/2 teaspoons salt and 1 teaspoon pepper. Toss to evenly coat.
Sear Beef: Turn the Instant Pot to the saute function. Once hot, add vegetable oil (2 tablespoons). When the display reads "hot", brown the beef in batches in a single layer, on all sides until deeply golden (don’t skip this step for flavor). Remove the beef to a plate. Add more oil if necessary to brown the rest of the beef.
Sauté Veggies: Melt the butter (2 tablespoons) in the pot. Add onions (1 cup), garlic (4 cloves), and mushrooms (8 ounces). Sauté until the mushrooms are golden brown and soft (about 5 minutes), stirring occasionally. This deep caramelization adds extra flavor.
Deglaze: Sprinkle flour (2 tablespoons) over the vegetables and stir to coat. Add the sherry (3 tablespoons) and 1/2 cup of beef broth to the pot. Stir and scrape the bottom to loosen all the browned bits. Let the liquid reduce for 2 minutes.
Build Sauce: Add the remaining beef broth (2 1/2 cups), soy sauce (2 tablespoons), Worcestershire sauce (1 tablespoon), Dijon mustard (1 tablespoon), tarragon (1/4 teaspoon), and thyme (1/2 teaspoon). Stir until combined. Add the browned beef and any juices back into the pot. Stir to combine.
Pressure Cook: Lock the lid and set the valve to seal. Set the Instant Pot to manual-high pressure for 15 minutes-the instant pot will start on its own. Once finished, perform a quick release by switching the valve to venting to quick release for 10 minutes.
Cook Noodles: After releasing the pressure, add the egg noodles (8 ounces) (or cook separately on the stovetop for firmer texture) directly into the Instant Pot. Set the Instant Pot to manual-high pressure for 3 minutes. Once finished, perform another quick release. Turn off the Instant Pot, remove the lid, and let it sit for 5 minutes.
Finish Sauce: Stir in sour cream (1 cup) (or a combination of sour cream and cream cheese). Adjust seasoning with salt and pepper, and add a pinch of smoked paprika (1/8 teaspoon) and lemon juice (if using) (1 teaspoon) for extra depth and brightness.👉 Pro Tip: Thickening the Sauce: If the sauce needs to thicken more, simply set the Instant Pot to the soup setting and stir occasionally until it thickens to your desired consistency. This gentle simmer will help the sauce reduce without overcooking the noodles. Serve: Ladle into individual bowls, sprinkle with parsley and serve.