Peel and thinly slice the onions. (save the roots and peels for making vegetable stock).
Heat the dutch oven over medium heat and add the olive oil. Add the onions and cook gently, stirring for about 5 minutes to separate the rings of the onion and get them coated in the oil.
Sprinkle on the salt and stir to combine. The salt will draw out the moisture in the onions. Cover with lid and reduce heat to medium low to continue cooking.
Stir the pot every 15-20 minutes to assure that nothing is sticking to the bottom. If you see the onions browning -- the pan is too hot and you should reduce the heat. You want the onions to slump over the course of cooking. When they've given up all of their liquid, only then do they start to caramelize. Don't confuse "brown" with "caramel".
Continue cooking until the liquid has evaporated and the onions take on a deep golden hue. The onions should be so tender that they almost fall apart and should be sweet.
The cooking time will vary on the pot your using and the stove. Just keep checking on it regularly, stirring when necessary and not letting it stick to the bottom of the pot.
5 pounds of onions reduces to about 2 cups of caramelized onions. If you're not using it all at once, portion out and freeze the remainder in small containers until ready to use.