How to Make It
Whisk together rice wine vinegar, sugar, and salt in a small bowl until dissolved. Add onion; chill at least 1 hour or up to 1 week.
Pour water to a depth of 1 inch in a large saucepan; bring to a boil over medium-high. Add clams and cook, shaking pot once or twice during cooking, until clams have opened, 7 to 10 minutes. Discard any unopened clams. Remove from pan, and let stand until cool enough to handle. Remove meat from shells.
Stir together cucumber, cilantro, hazelnuts, hazelnut oil, mint, Espelette pepper, salt, citrus segments, 3 tablespoons of the lime juice, and 2 tablespoons of the olive oil in a medium bowl. Stir in clam meat.
Whisk together remaining 1 tablespoon each olive oil and lime juice in a medium bowl. Add arugula, and toss to coat. Divide citrus salad evenly among 4 plates, and sprinkle with arugula mixture.