The trick: Cook the onions slowly, allowing the natural sugars to caramelize. Make several batches, and store in freezer.
Heat a large nonstick skillet over medium heat. Add sweet onions, and cook, stirring often, 30 minutes or until onions are caramel colored, sprinkling with salt halfway through.
Note: Store in refrigerator in an airtight container or zip-top plastic freezer bag up to 1 week, or freeze up to 2 months.