Chill this relish of zucchini, sweet onion, and bell peppers for up to 1 month.
Combine first 6 ingredients in a large bowl; toss to combine. Stir in 4 cups water; refrigerate 2 hours. Remove zucchini mixture from refrigerator; drain. Rinse and drain well.
Place zucchini mixture in a large skillet. Stir in vinegar and remaining ingredients; bring to a boil. Cook 15 minutes or until liquid almost evaporates, stirring occasionally. Remove zucchini mixture from pan; cool completely, and refrigerate.