I make a pot of these every now and then, portion the results ¼ cup at a time into freezer baggies, and store in the freezer for any time I want to add a little punch to a dish. Recipe is from kitchn.
3 to 5 pounds yellow onions (4 to 5 large onions)
3 T extra-virgin olive oil or melted butter
½ tsp salt
Peel and thinly slice the onions, and heap in your slow cooker until it's half to three-quarters full. Drizzle the olive oil or melted butter and salt over top. Toss evenly to coat.
Cover and cook for 10 hours on low. You can do this overnight, or you can do it all day, in which case go ahead and stir them occasionally. After 10 hours, they will be golden-brown and soft, and will have released a lot of liquid. You can stop here if you want. But I always keep going.
Cook 3-5 hours longer with the lid ajar so the liquid can evaporate, stirring every hour. When the onions have browned to your satisfaction and look and taste good to you, remove them with a slotted spoon and transfer to refrigerator or freezer containers. Any liquid remaining the the pot can be transferred to a separate container and used as a cooking broth or soup base.
Onions keep in the refrigerator for about a week or the freezer for several months.