How can I make a large amount of caramelized onions for a french onion soup?
You can caramelize onions in the oven to do large quantities.  This isn't less effort, and it actually takes longer, but it does allow you to do really large quantities; I did 20lbs of caramelized onions for one Polish dinner I was serving this way.  Since I haven't been able to find a website with this method, here it is:


heat oven to 325F/160C
fill a hotel pan or baking pan with sliced onions to a depth of about 3/4"/2cm
toss the sliced onions with vegetable oil or melted butter and a little salt (and optionally a little sugar)
cook, uncovered, in oven for 1 hour to 90 minutes, stirring every 10 minutes.


The reason for the 2cm depth of onions is to keep them from burning in the oven, which they tend to do if spread thinner.  Also, if you have a gas rather than electric oven, you may need to add a little water occasionally to prevent burning.

Althernately, Kenji of Serious Eats has a formula for cooking caramelized onions over high heat, and thus cooking each batch faster, without losing flavor.  This would allow you to jump through several batches.