Preheat an oven to 400 F, with the oven rack in the top 1/3 of the oven. Line a rimmed baking sheet with foil or parchment paper.
Make the meatballs: Use a box grater to grate the half onion into a mixing bowl, then add in the breadcrumbs and beaten egg. Let the mixture sit for 10 minutes. Add in the dijon, garlic, soup mix, and parsley and stir well. Add the ground sirloin and pepper and use your hands to mix well, just until everything comes together.
Bake the meatballs. Line a rimmed baking sheet with foil or parchment paper. Use a 3 tablespoon scoop to portion out the meatball mixture and roll them into balls. Place the meatballs on the prepared baking sheet. Bake for 10 minutes, then broil on high for 2-3 minutes until the meatballs are browned. Remove from the oven.
Brown the mushrooms. In a large braiser or skillet, heat the oil over medium-high heat. Once the oil is hot, add the sliced mushroom. Let them cook without moving for 2 minutes or until the mushrooms are brown on one side. Toss them and let them continue to brown for another 3-4 minutes.
Build the sauce. Lower the heat and add in the diced onion. Cook until the onion is softened, then stir in the butter and flour. Continue to cook for 2-3 minutes, stirring often, to cook out the raw flour flavor. Deglaze the pan with the brandy/sherry, then stir in the beef broth. Stir well so that no lumps remain and bring to a gentle simmer. Stir in the dijon mustard and Worcestershire sauce with the creme fraiche. Once the sauce is thick, season to taste with salt and pepper.
Simmer. Add the meatballs to the sauce, leaving any fat behind on the baking sheet. Baste the meatballs with the sauce. Reduce the heat to very low and gently simmer the meatballs for 10-15 minutes, stirring occasionally. Top with the chopped parsley. Serve the meatballs over buttered egg noodles or mashed potatoes.