Pat the ground beef dry with paper towels — this helps it brown instead of steam. Heat olive oil in a large skillet over medium-high heat. Add beef in an even layer and cook 5–7 minutes, breaking it up gently with a wooden spoon, until deeply browned and no pink remains. Drain excess fat if needed, but leave about 1 tablespoon for flavor.

To the same skillet, add diced onion and sauté 3 minutes until softened. Stir in sliced mushrooms and cook 4–5 minutes until they release their moisture and brown slightly. Add minced garlic and cook 30 seconds until fragrant. Sprinkle flour over the meat and vegetables, stirring constantly for 1 minute to cook off the raw flour taste.

Gradually whisk in beef broth and white wine, scraping up any browned bits from the bottom of the skillet. Stir in Worcestershire sauce, Dijon mustard, thyme, salt, and pepper. Transfer everything to the slow cooker insert, scraping the skillet clean with a wooden spoon.

Cover and cook on low for 7 hours. Do not lift the lid during this time — every time you open it, you lose valuable heat and extend the cook time.

After 7 hours, stir in heavy cream and cook uncovered on high for 15 minutes to gently warm through (do not boil). Remove from the slow cooker, stir in butter until melted for silkiness, and taste. Adjust salt or pepper if needed. Ladle over egg noodles or mashed potatoes, and sprinkle with fresh parsley.