Soak the red onion in a bowl of salted cold water, 10 to 15 minutes; drain, rinse and pat dry. Meanwhile, bring the vinegar and honey to a simmer in a small skillet over medium-high heat; cook until slightly syrupy, about 5 minutes. Let cool completely. (The syrup will thicken as it cools.) Transfer to a bowl and gradually whisk in the olive oil until emulsified.
Season the tomatoes with salt and pepper and arrange on a platter with the nectarines, arugula and onion. Top with the ricotta salata. Drizzle with the vinegar mixture and more olive oil; toss gently.