Combine chicken broth, beef broth, seasoning salt, pepper, garlic powder, and sugar. Bring to a boil; let simmer until reduced to about 6½ cups liquid.
Meanwhile, sauté onions in butter until half cooked.They'll be slightly yellow from the butter, soft and limber. Strain excess butter off.
Add onions to broth. Simmer for 10 minutes. Add Browning & Seasoning Sauce for color.
Simmer for another 5 minutes and then add sherry.
Serve hot.
Notes
Murray's recommended serving this soup in a tureen topped with a slice of garlic toast and sprinkled with Parmesan cheese.