Blanch the beans: Bring a pot of salted water to a boil. Add string beans and cook 3–4 minutes until just tender and bright green. Transfer to an ice bath, then drain and pat dry.
Make the vinaigrette: In a small bowl or jar, whisk together olive oil, mustard, honey, vinegar, salt, and pepper until emulsified.
Toast the nuts: In a dry skillet, toast nuts over medium heat for 3–4 minutes until fragrant. Cool slightly, then chop.
Assemble: Toss beans, onion, and tomatoes with half the vinaigrette. Top with toasted nuts and drizzle the remaining dressing just before serving.