1. Place a wire rack on a baking sheet. Arrange zucchini and squash slices on rack; sprinkle with 1 teaspoon salt. Let stand 30 minutes. Rinse well under cold water; pat dry with paper towels. Place zucchini, squash, and onion in a medium glass bowl.
2. Combine remaining 1 teaspoon salt, 1 cup water, and remaining ingredients in a small saucepan; bring to a boil, and pour over vegetables. Weigh down vegetables with a plate. Cover and refrigerate at least 2 hours or overnight. Drain or serve with a slotted spoon.
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