Preheat your oven to 400°F and position the oven rack in the top third. Line a rimmed baking sheet with foil or parchment paper.

Grate half an onion into a mixing bowl and combine with breadcrumbs and beaten egg. Let it sit for 10 minutes.

Add Dijon mustard, minced garlic, Lipton onion soup mix and fresh parsley to the mixture. Stir well.

Incorporate ground sirloin and cracked black pepper. Mix gently with your hands to avoid overworking the mixture.

Form the mixture into meatballs using a 3-tablespoon scoop and place them on the prepared baking sheet.

Bake meatballs for about 10 minutes and then broil on high for 2-3 minutes until browned. Remove from the oven.

In a large braiser or skillet, heat the neutral oil over medium-high. Add sliced mushrooms and cook undisturbed for 2 minutes until browned, then toss and cook for another 3-4 minutes.

Lower the heat and add diced onion, sauté until softened, then stir in butter and flour. Cook for 2-3 minutes stirring often.

Deglaze the pan with brandy or dry sherry, scraping up browned bits. Stir in beef broth and bring to a gentle simmer.

Add Dijon mustard, Worcestershire sauce, and crème fraîche. Allow sauce to thicken, seasoning with salt and pepper to taste.

Gently add meatballs to the sauce, basting them, and let everything simmer on low for 10-15 minutes. Top with chopped parsley before serving.