Bring a large pot of salted water to a boil and place a bowl of ice water nearby.
Add the green beans to the boiling water and blanch for 2 minutes. Using a slotted spoon or tongs, transfer the beans immediately to the ice water and leave until completely cool, about 1–2 minutes.
Drain the beans thoroughly and pat dry on paper towels.
In a large bowl, toss the cooled beans with the radicchio and the scant ¼ cup thinly sliced red onion. Add a few spoonfuls of the Honey Mustard Dressing to lightly coat the mixture, reserving additional dressing for drizzling.
Arrange the tossed mixture on a platter. Crumble the 2 ounces goat cheese into small dollops over the salad, then sprinkle with the 2 tablespoons chopped walnuts, 2 tablespoons sliced almonds, and the ¼ cup tarragon (roughly chopped if desired).
Drizzle with the remaining Honey Mustard Dressing, season to taste with flaky sea salt, and serve.