I love caramelized onions, but until now have made them only infrequently because of the time required to be spent standing in front of the stove to do the job properly. It seemed worth trying in the slow cooker, and I have to tell you, this is brilliant. You can make a whole big batch this way with minimal time spend having to tend them. That said, this is best done on a day when you'll be at home and can check in on the onions and stir them once in a while. Because slow cookers vary in shape and power, you'll need to see your particular one does here. My own is a large oval one, fairly new and pretty hot on the High setting.
Step 1: Fill your slow cooker with thinly sliced onions. Drizzle generously with olive oil and stir until the onions are all well coated.
The exact amount with vary with the size of your cooker. You don't have to make a full sized batch if you don't want to.
Step 2: Cook the onions on High, stirring once in a while. There will be a tendency for a few of the slices right at the edge to get brown and crispy and stick to the sides, so you'll want to stir them around periodically. This also gives you a chance to check on their progress. If your cooker runs especially hot on the High setting, you may want to switch it to Low after a few hours, or go back and forth. After 4 hours or so, my onions looked like this:
Continue cooking the onions, periodically stirring and scraping away from the edges. As they cooked down, I just pressed them inward a tiny bit from the sides of the cooker, which helped preventing sticking.
Step 3: Toward the end of cooking, 7-8 hours in or so, you'll see there's a lot of liquid in the cooker. To really properly caramelize, you need this to mostly evaporate. Leave the cooker on High, but crack open the lid. At this point my onions looked like this:
Step 4: Take the lid off entirely and continue to cook the onions on High, checking in periodically. You can use your judgment on when exactly to do this, but make sure it's a time when you will be able to watch them a little more more closely - checking in every 20-30 minutes instead of every few hours. When the onions are a nice rich shade of brown and the liquid is mostly gone, they're ready:
You can use them straight away, or put them away in the fridge or freezer.
Alternatively, you could leave them right in the cooker and add beef broth, white wine, salt and pepper to make the soup part of French Onion Soup. (Serve with a good quality peasant bread, toasted, and topped with melted cheese. Gruyere is traditional, but cheddar isn't bad.)