I'm designing a sandwich and I want to include a fruit relish as a topping. Are there methods that don't include pickling?
Fruit relishes (including salsas, chutneys, and plain sauces like applesauce) can be made without vinegar. But they almost always include some extra acid - if not vinegar, then usually citrus juice.
The acidity balances the sweetness of the fruit, and it preserves the color.
So you can make a fruit relish, substitute lemon or lime juice for the vinegar, and use it. But of course, without a full pickling process, the fruit relish will only have a limited life in the refrigerator.