1. In a food processor, make pesto by pulsing together tarragon, parsley, garlic, oil, cashews, pecorino, and pepper until coarsely chopped (not minced or paste-like). Set aside.
2. To a large pot filled with heavily-salted boiling water add potatoes, boil until tender, then remove and lay out on a sheet tray until cool. To the same water, add haricots verts, boil until just crisp-tender, shock in ice water, then drain and let cool.
3. Toss potatoes, cannellinis, haricots verts, and pesto together. Season with salt. Garnish with pickled red onions, if desired.