Step 1: Prepare the Ingredients

1. Slice the Bavette Steak: Trim any excess fat from the bavette steak and slice it thinly against the grain. Slicing against the grain keeps the meat tender, especially for cuts like bavette.
2. Chop the Vegetables: Thinly slice the onion and mushrooms. Mince the garlic and set aside.

Step 2: Sear the Steak

1. Heat the Oil: Heat 1 tablespoon of olive oil in a large skillet or sauté pan over medium-high heat.
2. Sear the Steak: Add the steak slices in a single layer and cook for 1-2 minutes per side, until browned but not fully cooked through. You may need to do this in batches to avoid overcrowding. Transfer the browned steak to a plate and set aside.

Step 3: Sauté the Vegetables

1. Add Butter to the Pan: Lower the heat to medium and add the butter to the same skillet.
2. Cook the Onions and Garlic: Add the sliced onions and cook for 3-4 minutes, stirring occasionally until softened and lightly golden. Add the minced garlic and cook for an additional 30 seconds, until fragrant.
3. Add the Mushrooms: Stir in the mushrooms and cook for about 5-7 minutes, or until the mushrooms are golden brown and any released liquid has evaporated.

Step 4: Build the Sauce

1. Sprinkle with Flour: Sprinkle 1 tablespoon of flour over the vegetables and stir to coat. This helps thicken the sauce.
2. Add Broth and Worcestershire Sauce: Gradually pour in the beef broth, stirring continuously to avoid lumps. Add the Worcestershire sauce and bring the mixture to a simmer.
3. Simmer to Thicken: Let the sauce simmer for 5-6 minutes, or until it begins to thicken slightly.

Step 5: Finish the Sauce

1. Add Sour Cream and Dijon Mustard: Reduce the heat to low and stir in the sour cream, heavy cream, and Dijon mustard until smooth. This will give the sauce a creamy texture and a slight tanginess.
2. Season with Salt and Pepper: Taste the sauce and season with salt and pepper as needed.

Step 6: Combine and Serve

1. Add Steak Back to the Pan: Return the seared bavette steak to the skillet, stirring gently to coat the meat in the sauce. Allow it to cook for an additional 2-3 minutes, or until the beef is warmed through.
2. Serve: Serve the stroganoff over a bed of egg noodles, rice, or mashed potatoes. Garnish with chopped fresh parsley, thyme, or grated Parmesan for an elegant finish.