Mushroom Mixture: Heat 1 Tbsp. oil in large nonstick skillet over medium heat and cook onion with mushrooms, stirring occasionally, until tender, about 4 minutes. Stir in Knorr® Beef flavor Bouillon Cube and cook 1 minute. Remove from heat, cool slightly and reserve 2 Tbsp of mixture.
Combine ground beef, bread crumbs, egg and larger portion of mushroom mixture in large bowl. Shape into rectangular loaf, about 1-inch-thick on aluminum-foil lined baking sheet. Brush all sides of the loaf with remaining oil. Invert meatloaf onto pre-heated grill; remove foil. Cook over medium-high heat until well marked, about 4 minutes. Gently turn and cook an additional 4 minutes. Turn off burners directly under meatloaf, keeping other burners on low. Close grill cover and cook until meat reaches 160°F, turning or rotating meat loaf as necessary to brown evenly, about 10 minutes. Allow to stand 5 minutes
Prepare Knorr ® Gravy Classics ™ Classic Brown Gravy Mix according to package directions, stirring in 2 Tbsp. mushroom mixture and sour cream. Serve with meatloaf.