Line a 13- x 9-inch or similar sized baking dish with foil. Preheat oven to 350 degrees F.
Add olive oil and butter to a 12-inch skillet and place over MEDIUM heat. Add mushrooms, diced onion, parsley, and garlic and saute for a couple of minutes, just until slightly softened. Season with 1/2 teaspoon salt and fresh ground black pepper, to taste. Remove from heat and set aside.
Combine ground beef, beaten eggs, ketchup, bread crumbs, mustard, Worcstershire sauce, and seasoned salt in a large mixing bowl. Add half of the cooked and slightly cooled mushroom mixture and combine well. Scrape the mixture into the foil-lined baking dish and use your hands to form it into a football shape.
Transfer remaining cooked mushroom mixture to a small bowl and refrigerate for later.
Bake meatloaf for 50 to 60 minutes, or until it reaches an internal temperature of 160 degrees F. The meatloaf should be cooked through with no pink remaining when cut through the center. Remove dish from oven and and allow meatloaf to rest while you make the sauce.
Add beef broth, Dijon, Worcestershire sauce and reserved cooked mushroom mixture to a deep skillet and place over MEDIUM heat. Combine cornstarch and water in a small bowl and stir the slurry into the sauce until thickened. Stir in the sour cream and continue to cook on LOW until warmed through. Taste and season with additional salt and fresh ground black pepper, if desired.
Carefully transfer meatloaf to a serving platter or cutting board. Serve sliced meatloaf with a ladle of sauce.