Here's a recipe for onion soup for one and quite honestly, after making it, I'd suggest you just make a big batch of it because it's so good! (Thank you to the sister who made it and inspired me to try it also!)
*NOTE: I forgot to note 'hot' broth.
And another note...I cooked my onions on the woodstove (for free) and living the way we do I wouldn't use 20 minutes of propane (we use a little tiny propane cooker) just for one serving of soup so you might as well make a big one and have several meals.
Praises to my sisters who do their good works unto the Lord, serving others.