Heat olive oil in a large skillet over medium heat.
Add the onion and garlic, cooking until softened and fragrant.
Stir in the ground beef and cook until browned all over.
Add the mushrooms and cook until they release their juices and begin to brown.
Stir in both mustards, coating the beef and mushrooms evenly.
Pour in the beef stock and bring to a gentle simmer.
Add the cornstarch slurry and stir until the sauce thickens.
Season with salt and pepper.
Serve hot, garnished with fresh parsley.