Rinse and drain black-eyed peas. Place in medium heavy saucepan and add onion halves (with cloves), bay leaf and enough water to cover by an inch. Bring to the boil, reduce to barely a simmer, cover and cook 20 minutes. Add 1 teaspoon salt and 1/2 teaspoon ground pepper, and a little more water if none is visible when you tilt the pan, and cook, covered, another 20-30 minutes until beans are creamy inside but still intact. Drain and remove onions and bay leaf. Let cool.

Meanwhile, bring large pot of water to the boil. Add 2 tablespoons salt and 5 corn cobs. Cook corn 23 minutes. Remove corn and repeat with remaining cobs. Let cobs cool, then carefully cut corn from cob with sharp knife. Place cut corn in a medium bowl and add diced zucchini and squash, tomatoes, and scallions. Place vinegar in a small bowl and whisk in yogurt along with 1 teaspoon salt and 1/2 teaspoon pepper. Drizzle in olive oil while whisking so as to form an emulsion. Pour dressing over corn and vegetables. Add cooled black-eyed peas and chopped parsley. Stir gently to combine and serve.