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 overbrowning).
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 marmalade warm with cooked chicken or pork, or spread on toasted baguette slices. If desired, transfer to a medium bowl; cover and refrigerate up to 5 days. Warm marmalade in saucepan before serving. Makes 2-1/2 cups (twenty 2-tablespoon servings).