Before starting, slice your beef sirloin across the grain into ¼-inch thick pieces. For easier slicing, place the meat in the freezer for 20 minutes first. Season the sliced beef generously with salt and pepper and let it sit at room temperature for 15 minutes.
Heat your large pan over medium-high heat (350°F). Add 1 tablespoon of oil and sauté the mushrooms for about 30 seconds until lightly browned. Remove the mushrooms and set aside.
In the same pan, add another tablespoon of oil if needed. Turn the heat to high (400°F). Place the beef slices in a single layer and sear for 1-1.5 minutes on each side until nicely browned. Don't overcrowd the pan – cook in batches if needed. Remove the beef and set aside.
Lower the heat to medium (320°F) and add the butter to the pan. Once melted, add your chopped onions and cook for 5 minutes until soft and clear. Add the minced garlic and cook for another minute until you can smell the garlic.
Sprinkle the flour over the onions and garlic. Stir continuously for 1-2 minutes to cook the flour. This prevents your sauce from tasting like raw flour.
Slowly pour in the beef broth while whisking constantly to prevent lumps from forming. Keep whisking until the sauce is smooth.
Return the beef to the pan. Bring everything to a gentle boil, then reduce the heat to low (285°F). Cover the pan and let it simmer for about an hour to an hour and a half, or until the beef is tender enough to cut with a fork. If the sauce gets too thick before the meat is tender, add warm beef broth ½ cup at a time.
Once the meat is tender, pour in the cream while stirring gently. Add the mushrooms back to the pan. Let everything simmer uncovered for 2-3 minutes until the mushrooms are heated through and the sauce reaches your desired thickness.
Taste the sauce and add more salt and pepper if needed. Turn off the heat and let it rest for 5 minutes before serving. The sauce will continue to thicken slightly as it cools.
Serve hot over steaming rice, mashed potatoes, or egg noodles. Garnish with fresh parsley if desired.