How to Make It
Preheat the oven to 400°. Peel the onions, leaving the root ends intact. Cut off the onion tops and scoop out the centers with a spoon or melon baller, leaving the outer 2 layers intact. Arrange the hollowed-out onions in a large baking dish and season them well inside and out with salt and pepper. Pour the chicken stock into the baking dish, cover with foil and bake for about 35 minutes, or until the onions are tender. Using a slotted spoon, transfer the onions to a large platter and let cool.
Meanwhile, in a small dry skillet, toast the white sesame seeds over moderately high heat, stirring until lightly browned, about 2 minutes; let cool. In a small bowl, whisk together the vinegar, olive oil, sesame oil, white and black sesame seeds and a pinch of salt.
In a large saucepan of lightly salted boiling water, cook the green beans until just tender, about 6 minutes. Drain the beans, cool under running water and drain again; pat dry with paper towels. Cut the beans into 1-inch lengths and transfer them to a large bowl. Add the scallions, red onion and sesame dressing, season with salt and toss. Fill the onions with the bean salad and serve.