In a large bowl, combine the ground pork, ground beef, breadcrumbs, 1/2 finely minced onion, egg, 2 teaspoons salt, and 1 teaspoon black pepper. Mix gently until just combined to avoid tough meatballs, then roll the mixture into approximately 25 uniform meatballs. Separately, dice the remaining whole onion and mince the garlic cloves for later use. This mise en place ensures a smooth cooking process.

Heat 1 tablespoon of vegetable oil in a large skillet or Dutch oven over medium-high heat. Brown the meatballs in batches on all sides until nicely caramelized. Remove the browned meatballs and set them aside. Add the 1 tablespoon butter to the same pan, then add the sliced mushrooms. Sauté until they release their liquid and turn golden brown – I find this step really builds a deep, earthy flavor base for the sauce. Once mushrooms are browned, add the diced onion and cook until softened, about 5 minutes.

Stir in the minced garlic and cook for just 30 seconds until fragrant, being careful not to burn it. Sprinkle the 2 tablespoons of flour into the pan and cook, stirring constantly, for about 2 minutes to create a roux. This step is crucial for thickening the sauce. Deglaze the pan with the brandy, if using, scraping up any browned bits from the bottom of the pan, and let it reduce by half. Add the 2 cups of beef stock, 1/2 cup beef stock, whole grain mustard, 1/2 teaspoon salt, 1/2 teaspoon black pepper, and bay leaf, then bring the sauce to a simmer.

Return the seared meatballs to the simmering sauce, ensuring they are mostly submerged. Reduce the heat to low, cover the pan, and let it gently simmer for 15 minutes, stirring occasionally. This allows the meatballs to cook through and absorb the delicious flavors of the sauce. After simmering, remove the pan from the heat and stir in the room temperature sour cream. I always make sure the sour cream is at room temperature to prevent it from curdling.

Remove the bay leaf before serving. Garnish the Meatball Stroganoff generously with fresh parsley. This dish is wonderful served over egg noodles, rice, or even mashed potatoes. Enjoy your rich and comforting meal!