Ingredients :

• Ground beef — Avoid 5% fat — too dry, as the meatballs crumble during cooking. 15-20% fat holds much better. A beef-veal mix gives something softer if you can find it.
• Button mushrooms — White or brown, both work. Brown (cremini) have a slightly more pronounced flavor. The key: don’t slice them too thin so they stay present in the sauce once cooked.
• Full-fat sour cream or crème fraîche — No liquid cream here. Thick cream holds up to heat and gives that coating texture. Look for at least 30% fat.
• Dijon mustard — Just one spoonful is enough. You won’t taste it directly in the sauce, but it adds depth and rounds everything out. No need to add more.