Step 1: Prepare the Meatballs

1. Combine Wet Ingredients: In a large mixing bowl, gently whisk together the beaten egg, milk (or buttermilk), finely minced onion, minced garlic, chopped parsley, Worcestershire sauce, salt, and pepper.
2. Add Breadcrumbs: Stir in the panko breadcrumbs and let the mixture sit for 5-10 minutes. This allows the breadcrumbs to absorb the liquid, which helps keep the meatballs moist and tender.
3. Add Ground Beef: Add the ground beef to the bowl. Using your hands, gently mix all the ingredients until just combined. Be careful not to overmix, as this can result in tough meatballs. Overmixing develops the gluten in the meat, leading to a denser, chewier texture. The goal is a tender, light meatball.
4. Form Meatballs: Lightly wet your hands to prevent sticking. Roll the mixture into meatballs approximately 1.5 inches in diameter. You should get about 20-24 meatballs. Place them on a baking sheet lined with parchment paper as you form them.

Step 2: Cook the Meatballs

You have a couple of options here – pan-frying for a crispier exterior or baking for a more hands-off approach.

• Pan-Frying (Recommended for best flavor and texture):

  1. Heat 2 tablespoons of olive oil in a large, heavy-bottomed skillet or Dutch oven over medium-high heat. The oil should shimmer.
  2. Carefully place the meatballs in the hot skillet in a single layer, ensuring not to overcrowd the pan (work in batches if necessary).
  3. Brown the meatballs on all sides, turning them gently, for about 5-7 minutes total per batch. They don’t need to be cooked through at this stage, as they will finish cooking in the sauce.
  4. Remove the browned meatballs from the skillet and set them aside on a clean plate. Do not wipe out the skillet; the browned bits (fond) left behind are packed with flavor and will enrich your sauce.

• Baking (Easier, less mess):

  1. Preheat your oven to 400°F (200°C).
  2. Arrange the meatballs in a single layer on a baking sheet lined with parchment paper or a lightly greased baking dish.
  3. Bake for 15-20 minutes, or until they are nicely browned and mostly cooked through.

Step 3: Sauté the Aromatics

1. Cook Onions: To the same skillet used for the meatballs (if pan-fried, there might be enough residual fat; if not, or if you baked the meatballs, add 1-2 tablespoons of butter or oil), add the sliced yellow onion. Sauté over medium heat for 5-7 minutes, stirring occasionally, until softened and translucent. If the pan seems dry, add a bit more butter.
2. Add Mushrooms: Increase the heat slightly to medium-high. Add the sliced mushrooms and the remaining butter (if needed) to the skillet. Cook, stirring occasionally, for 8-10 minutes, until the mushrooms have released their liquid, the liquid has evaporated, and the mushrooms are nicely browned and slightly caramelized. This browning step is crucial for developing a deep, earthy flavor.
3. Add Garlic: Stir in the minced garlic and cook for another 1-2 minutes until fragrant. Be careful not to burn the garlic, as it can become bitter.

Step 4: Build the Sauce Base

1. Make the Roux: Sprinkle the all-purpose flour over the cooked vegetables. Stir constantly and cook for 1-2 minutes. This toasts the flour, removes its raw taste, and creates a roux that will thicken the sauce.
2. Deglaze and Add Broth: Gradually whisk in the beef broth, a little at a time, scraping up any browned bits from the bottom of the pan. This process, called deglazing, incorporates all those delicious flavors into your sauce.
3. Add Seasonings: Stir in the Dijon mustard, Worcestershire sauce, and smoked paprika (if using). Bring the mixture to a gentle simmer, whisking until the sauce begins to thicken slightly.

Step 5: Simmer and Meld Flavors

1. Return Meatballs to Sauce: Gently add the browned meatballs (and any accumulated juices from the plate) back into the simmering sauce. Ensure they are mostly submerged.
2. Simmer: Reduce the heat to low, cover the skillet, and let the meatballs simmer gently in the sauce for 15-20 minutes. This allows the meatballs to cook through completely and absorb the flavors of the sauce, while the sauce itself continues to thicken and its flavors meld. Stir occasionally to prevent sticking.

Step 6: The Creamy Finish

1. Temper Sour Cream: This is a crucial step to prevent the sour cream from curdling when added to the hot sauce. In a small bowl, whisk the room-temperature sour cream. Gradually ladle about ½ cup of the hot sauce from the skillet into the sour cream, whisking constantly until smooth.
2. Incorporate into Sauce: Remove the skillet from the heat. Pour the tempered sour cream mixture into the skillet with the meatballs and sauce. Stir gently until well combined and the sauce is smooth and creamy. Do not bring the sauce back to a boil after adding the sour cream, as this can cause it to curdle or separate.
3. Taste and Adjust: Taste the sauce and adjust seasoning with salt and freshly ground black pepper as needed. You might want a little more Dijon or Worcestershire depending on your preference.

Step 7: Serve and Garnish

1. Serve Hot: Ladle the Meatball Stroganoff generously over hot cooked egg noodles, mashed potatoes, or rice.
2. Garnish: Sprinkle generously with fresh chopped parsley or dill for a touch of color and freshness.