Heat a large heavy skillet over high heat. I like to use my giant cast iron. Spray with a quick mist of olive oil spray and let it get nice and hot.
Sprinkle chopped steak with kosher salt and cracked black pepper. Then add it to the hot pan in a single layer. Let it cook undisturbed and uncrowded for a minute or two until browned, salt and pepper the tops (if you haven’t already) before you flip. Then cook for just a minute on the other side. Remove from the pan and set aside.
Add onions and cook for a minute, then add mushrooms and cook for about 5 minutes or until the onions and mushrooms have cooked down and are very soft. Add garlic, dijon mustard, thyme, and flour. Toss the flour around well and cook for just a minute.
Add broth and deglaze the pan, scraping all the browned bits off the bottom. Bring the mixture to a boil and then reduce heat and simmer for about 5 minutes to let it thicken.
Add a few Tbsp of the hot beef broth to the Greek yogurt to warm it up a bit (this will help it from separating when it's mixed into the hot broth), then stir the Greek yogurt into your sauce. Add back in steak and combine well. Garnish with chopped green onion, chives, or parsley.
Serve over egg noodles for a more authentic stroganoff, or use whatever pasta your kids love! I love mine over roasted spaghetti squash and broccoli.