In a 12-inch skillet cook onions, uncovered, in hot oil over medium heat about 20 minutes or until very tender and beginning to brown, stirring occasionally (reduce heat to medium-low, if necessary, to prevent over browning).
Add undrained pineapple, vinegar, granulated sugar, brown sugar, and salt to skillet.
Bring to boiling, stirring to dissolve sugars; reduce heat.
Simmer, uncovered, for 35 to 45 minutes or until thickened to desired consistency, stirring occasionally.
Serve warm or transfer to a medium bowl; cover and refrigerate up to 5 days. Warm marmalade in saucepan before serving.