Place a large, deep skillet or Dutch oven over medium-high heat. Pour in the olive oil and allow it to become very hot.
Arrange the sliced sirloin in a single layer. Sear for approximately 1 minute on each side without stirring, cooking only until the exterior is browned and the meat is no longer red. Work in two batches when necessary to prevent overcrowding.
Transfer the browned beef to a plate and cover it to retain its warmth.
Add the unsalted butter to the same pan. Once melted, add the finely chopped onion and sliced mushrooms.
Cook for 6–8 minutes, stirring from time to time, until the vegetables are tender and lightly browned and the moisture released by the mushrooms has evaporated.
Stir the minced garlic into the cooked onion and mushrooms and sauté for 1 minute, just until aromatic.
Scatter the all-purpose flour evenly over the vegetables. Continue cooking for another minute while stirring continuously so the flour cooks into the mixture and can later help thicken the sauce.
Stir in the heavy cream and allow the mixture to simmer for 1–2 minutes, until it begins to develop a slightly thicker consistency.
Place the sour cream in a small bowl and blend in several tablespoons of the hot sauce. Once the sour cream has been tempered, gradually stir it back into the pan while continuously mixing to help prevent curdling.
Add the Worcestershire sauce, Dijon mustard, salt, and black pepper. Stir until the seasonings are thoroughly incorporated and the sauce is creamy and uniform.
Return the seared sirloin to the pan along with any juices that have collected on the plate. Bring the stroganoff just to a gentle simmer and cook only until the beef is warmed through.
Taste the finished sauce and adjust the salt or pepper as desired.
Spoon the hot beef stroganoff over cooked egg noodles, if using.
Finish with chopped green onions or fresh parsley and serve immediately.