Heat a tablespoon of olive oil in a large skillet over medium-high heat. Add the thinly sliced beef sirloin in batches, searing for about 3-5 minutes until browned on all sides. Remove and keep warm.

In the same skillet, lower the heat and add the chopped onion and minced garlic. Sauté for 2-3 minutes until translucent. Add sliced mushrooms and cook for another 4-5 minutes until golden brown.

Return the seared beef to the skillet, add 1 cup of beef broth and 2 tablespoons of Dijon mustard. Increase heat, scrape the skillet's bottom, and simmer for about 5 minutes.

Remove from heat and stir in 1 cup of sour cream until creamy. Season with salt and pepper to taste.

Prepare your chosen base. Spoon beef stroganoff over it and serve hot.