In a large bowl, stir together panko and milk. Let soak for 2–3 minutes until spongy.
Add ground beef, egg, onion, Worcestershire sauce, Dijon, salt, and pepper. Mix gently just until combined.
Scoop a heaping tablespoon of the mixture using a cookie scoop or spoon and roll into meatballs. Lightly wet your hands if needed to prevent sticking.
You should end up with about 18-20 meatballs.
Heat olive oil in a large skillet over medium-high heat. Brown the meatballs for about 6-8 minutes, turning occasionally, until browned on all sides (they don’t need to be cooked through). Remove to a plate and set aside.
To make the stroganoff sauce:
In the same skillet, melt butter over medium heat. Add onion and cook until softened, about 2 minutes. Stir in garlic and cook for 30 seconds more. Add mushrooms and thyme and cook, stirring occasionally, until the mushrooms release their moisture and begin to turn golden, about 5-6 minutes.
Sprinkle flour over the mushrooms and stir well. Cook for 1 minute.
Add white wine, scraping up any browned bits from the bottom of the skillet. Let it cook for 1-2 minutes, until slightly reduced. Stir in beef broth, Dijon mustard, and Worcestershire sauce. Bring to a simmer and cook until the sauce begins to thicken, 3-5 minutes.
Turn the heat to low and stir in sour cream until smooth.
Add the meatballs back to the skillet and simmer until cooked through, about 8-10 minutes.
Taste and add salt and pepper as needed.
Garnish with chopped parsley and serve over mashed potatoes or egg noodles.
Notes
Don’t worry about cooking the meatballs all the way through when browning, since they finish cooking in the sauce.
Use low-sodium beef broth if possible. Worcestershire, Dijon, and the meatballs already add salt, so this helps keep the dish from getting too salty.
Stir in the sour cream over low heat and don’t let the sauce boil. This keeps the sauce smooth and prevents it from separating.
Store leftover meatballs and sauce together in an airtight container in the refrigerator for up to 3 days. Reheat on the stove over low heat or in the microwave. If the sauce has thickened, add a splash of broth or water to thin it out.
This dish can also be frozen in a freezer-safe container for up to 2 months, but the sauce may separate after thawing. Stir well while reheating to help the sauce come back together.