Start by slicing the beef sirloin or tenderloin into thin strips, cutting against the grain for maximum tenderness. Clean and slice your mushrooms, mince the garlic, and finely chop the onion to build layers of flavor.

Heat 1 tablespoon of olive oil and 1 tablespoon of butter in a skillet over medium-high heat. Sear the beef in batches for 1-2 minutes on each side, until browned. Set the beef aside to retain its juices.

Reduce the heat to medium and add the remaining butter to the skillet. Sauté the chopped onions for 3-4 minutes until they become translucent. Next, add the mushrooms and cook until they’re browned and tender. Stir in the minced garlic, cooking for an additional 30 seconds to release its aromatic goodness.

Sprinkle the flour evenly over the sautéed vegetables, mixing well to coat. Gradually whisk in the beef broth while ensuring there are no lumps. Add the Dijon mustard, letting the mixture simmer for about 3-4 minutes until it thickens slightly.

Lower the heat and gently stir in the sour cream, blending it into the sauce until smooth. Return the seared beef and any accumulated juices back to the skillet, warming through for 2-3 minutes without boiling to keep the sauce creamy.

Taste your delicious creation, adjusting the seasoning with salt and pepper as desired. Serve hot, over buttered egg noodles, rice, or mashed potatoes, and garnish with fresh parsley for a pop of color.