Preheat oven to 400°F (200°C) and place French bread sections on a baking sheet cut-side up.
Heat olive oil in a large skillet over medium-high heat and sear beef strips until browned; remove beef from skillet and set aside.
In the same skillet, melt butter and sauté mushrooms and onions for 6-8 minutes until softened and moisture has evaporated.
Add garlic and flour to the skillet and cook for 1 minute while stirring constantly.
Slowly whisk in beef broth and Worcestershire sauce, simmering until the mixture thickens into a gravy.
Reduce heat to low, return beef to the skillet, and fold in the sour cream until well combined. Season with salt and pepper.
Toast the French bread in the oven for 5 minutes until lightly golden.
Spoon the beef stroganoff mixture evenly onto the toasted bread sections and top with shredded mozzarella cheese.
Bake for an additional 5 minutes until the cheese is melted and bubbly.
Garnish with fresh parsley and serve immediately.