Step 1: Heat olive oil in a large skillet or Dutch oven over medium-high heat. Add ground beef and cook, breaking it up with a spoon, until browned. Drain any excess fat and set the beef aside.
Step 2: Reduce heat to medium. Add chopped onion to the skillet and cook until softened, about 3-4 minutes. Add sliced mushrooms and cook until they release their liquid and start to brown, about 5-7 minutes. Stir in minced garlic, dried thyme, salt, and pepper; cook for 1 minute until fragrant.
Step 3: Sprinkle flour over the onion and mushroom mixture. Cook, stirring constantly, for 1-2 minutes to create a roux. This helps thicken the sauce.
Step 4: Gradually whisk in the beef broth, scraping up any browned bits from the bottom of the pan. Bring the mixture to a simmer, then reduce heat to low and cook until the sauce thickens slightly, about 3-5 minutes. Stir in the Dijon mustard.
Step 5: Return the cooked ground beef to the skillet. Remove the skillet from the heat and stir in the sour cream until well combined and creamy. Do not boil after adding sour cream, as it may curdle. Taste and adjust seasoning as needed.
Step 6: Serve immediately over hot egg noodles, rice, or mashed potatoes. Garnish with fresh chopped parsley. Enjoy your delicious ground beef stroganoff 20 minute cozy dinner!