Begin by heating the olive oil in a non-stick frying pan over medium heat. Add the sliced onion and gently cook, stirring occasionally, until completely softened and sweetly fragrant, about 15 minutes. If the onions begin to stick, add a small splash of water to keep them from burning — this slow cooking step enhances their natural sweetness. Next, crush in the garlic and allow it to cook for 2-3 minutes until aromatic. Introduce the butter to the pan; as it foams gently, add the mushrooms. Cook these until they release their moisture and become tender, approximately 5 minutes. Season the mixture generously with salt and pepper to enhance all the flavors, then transfer the onions and mushrooms to a plate for later. In a separate bowl, combine the plain flour with a big pinch of salt and pepper. Toss the beef fillet pieces in this seasoned flour, coating them lightly — this will give the beef a lovely crust when fried. Add the beef to the pan, adding a tiny splash of oil if the pan looks dry, and fry for 3-4 minutes until the pieces are beautifully browned. Return the onions and mushrooms to the pan with the beef. In a small bowl, whisk together the crème fraîche, English mustard, and beef stock until smooth, then pour this luscious mixture into the pan. Simmer over medium heat for about 5 minutes, allowing the sauce to thicken and the flavors to meld. Finish by scattering fresh parsley over the stroganoff for a touch of color and herbal brightness. Serve your beef stroganoff piping hot alongside pappardelle pasta or steaming rice to soak up every last drop of sauce. Enjoy this comforting and elegant meal with your favorite side!