- Preheat oven to 425°F. Spray large baking sheet with olive oil nonstick spray.
- Combine bell peppers and onion in large bowl; spray with nonstick spray and toss until coated. Add oil, rosemary, salt, and black pepper; toss until coated evenly.
- Spoon vegetable mixture onto prepared baking sheet and spread to form even layer. Roast 10 minutes; turn vegetables. Roast until browned in spots and tender, 10–15 minutes.
- Per serving: 3/4 cup
Make it ahead. Prepare this dish as directed. Transfer to an airtight container and let cool. Cover and refrigerate up to 3 days. To serve warm, transfer the vegetable mixture to a microwavable dish. Cover with a sheet of wax paper and microwave on High, stirring several times, just until warmed through, about 3 minutes. To serve at room temperature, let the dish stand out up to 2 hours.