Georgia's Vidalia and Texas's 1015 sweet onions are available in spring and summer, but sweet onions are now available just about all year long. In fall and winter months, look for the South American OSO sweet onion.
Cook onion and thyme in hot oil in a large deep skillet over medium heat, stirring often, 35 to 40 minutes or until caramel colored (a deep golden brown). Remove from heat; stir in salt.
Note: Store cooked onions in a zip-top plastic freezer bag or an airtight container in refrigerator up to 1 week or freeze up to 2 months.