This recipe calls for a large, underripe heirloom tomato to give it a firm texture that will stand up to the pickled squash. A yellow or orange heirloom tomato will give it a particularly sunny color, but an underripe heirloom of any color should taste just as nice.
1. Whisk together vinegar and 2 Tbs. water in small bowl. Add squash, season with salt (if desired), and let stand 10 minutes.
2. Meanwhile, stir together tomato, onion, and oil in bowl.
3. Drain squash well; stir into diced tomato mixture. Season with salt and pepper, if desired. Let stand 5 to 10 minutes.