Heat olive oil in a large saucepan over high heat. Sear the beef for about 5 minutes, flipping half way so that both sides of the meat are browned. Season with salt and pepper.
Add in the chopped onion and garlic, tossing everything together for about 3 minutes, until onion is soft.
Pour in the beef broth and make sure to scrape off any of browned bits from the bottom of the pan.
Add in the sliced mushrooms, stir together and turn heat to low. Let everything simmer for 30 minutes, stirring occasionally.
In a separate bowl, mix together sour cream, dijon mustard, and xanthan gum. Ladle out about 1/2 cup of the hot beef broth and slowly stir it into the sour cream mixture. (This step is to prevent the sour cream from curdling.)
Next, pour the sour cream mixture in with the meat and stir until it is fully incorporated. Let everything simmer for a few minutes until sauce has thickened.
Serve Stroganoff over zucchini noodles and sprinkle with chives, if desired.
This recipe makes approximately 7 cups and each serving is about 1 cup each. Macros do not include the zucchini noodles. For leftovers, store in the fridge and enjoy within 4 days.