Place vinegar and rosemary in a blender and blend until smooth.
Slowly add olive oil until emulsified and season with salt and pepper to taste. Strain into a bowl and preheat grill.
Toss potatoes in a large bowl with olive oil and season with salt and pepper to taste.
Grill skin side down until golden; turn over and continue grilling until just cooked through.
Place in a large bowl and immediately toss with 1/4 cup vinaigrette.
Meanwhile, bring a large pot of salted water to a boil and add haricots verts. Cook for 2 minutes and immediately plunge into a bowl of ice water.
Drain – then add haricots verts and red onion to potatoes and drizzle with remaining vinaigrette. Season with salt and pepper to taste.