6 lbs yellow onions peeled & trimmed
cooking spray
3 tbsp olive oil
Spray the ceramic insert of a 4- or 5-quart slow cooker.
Thinly slice the onions (a food processor makes quick work of this), and pile the onions into the slow cooker. Drizzle with 2-3 tablespoons of olive oil, and stir with a rubber spatula. The cooker will seem impossibly full, but that's fine.
Cover and cook on HIGH for 8 hours, stirring 2 or 3 times. The onions will reduce to fill less than a third of the cooker, and will turn golden brown and soft. Then, uncover the cooker, and continue to cook on HIGH for 1 hour, to allow some of the liquid to evaporate.
Remove the cooked onions from the slow cooker, let cool, and pack into containers with tight-fitting lids. Refrigerate for up to 2 weeks.