Preheat oven to 400 degrees F.
Cut off the stem and root ends of the onions, then halve then lengthwise, and peel away the dried brown skin. Cut the onions into 1/4-inch slices. It is important to have all the onion slices roughly the same size so they cook evenly.
Place all the onion slices in a large, heavy pot or Dutch oven (I like to spray the inside of my pot with non-stick vegetable spray).
At this point, your pot will be full of sliced onions, but the onions will wilt and shrink dramatically during the cooking process. Toss the sliced onions with the vegetable oil and salt to help draw out the moisture and to keep them from sticking to the pot. Cover the pot and place in the preheated oven.
Set your timer for 15 minutes. After 15 minutes, open the oven, remove the lid, and the stir the onions well (scrape all sides of the pot and make sure you get all the bits and pieces - stir the onions well). Cover with lid and return to the oven. Repeat this 15 minutes process for approximately 2 to 2 1/2 hours. Watch the onions shrink and the color change to a reddish-brown.
After you reach the 2-hour point, your onions will have almost no water left and will have darkened and taste sweet. NOTE: If you feel you have too much water left in the pot, place the pot back in the oven with no lid for an additional 15 minutes to evaporate the excess water.
To store the caramelized onions, store in an airtight container in the refrigerator for up to 2 weeks or they can be frozen for up to 3 months.
Optional Idea: Add a little Balsamic Vinegar during the last 15 minute cycle. The balsamic vinegar adds a little sweetness and balances it with some tartness.
Makes approximately 2 1/2 cups.