Put onion in food processor and pulse until finely chopped. Transfer to a bowl. Wipe food processor clean, add bell pepper, and pulse until coarsely chopped.
Heat oil over medium heat in a wide 8-qt. pot. Add spices and cook, stirring frequently, until toasted and aromatic, about 3 minutes. Add onion and bell pepper, stirring to combine. Stir in sugar, vinegar, zucchini, salt, and 1/2 cup water and cook, covered, stirring often, until zucchini has softened and flavors have melded, about 20 minutes.
Fill 7 half-pt. jars and put lids on to seal. Relish keeps, chilled airtight, at least 2 weeks.