Place the onion in a small bowl. Cover with cold water and let stand for 30 minutes. Drain and pat dry with clean kitchen towels; set aside.
Prepare an ice water bath; set aside. Bring a large pot of water to a boil. Add a generous amount of salt and the haricots verts. Cook until bright green, about 2 minutes. Use a slotted spoon to transfer to the ice water bath to stop cooking. Drain, shaking off excess liquid; set aside.
In a large bowl, whisk together the oil, vinegar, salt, and pepper. Add the haricots verts and toss to combine. Gently toss in tomatoes and onion slices. Transfer to a serving platter. Garnish with ricotta salata.