Mince the shallot and place in a small bowl along with the rice vinegar and salt. Let the minced shallot rest in the vinegar (this will help mellow the raw shallot taste a bit).
Preheat the oven to 300˚F. Roughly chop the walnuts, place on a small sheet tray, and then into the oven. Roast until the nuts are fragrant and slightly browning, 10-12 minutes. Remove from the oven and set sides.
Place a large skillet over high heat with the sunflower oil (and turn on the vent if you have one). Once the skillet is hot, add the green beans. Sear the green beans until blistering, stirring only a couple of times. Searing will go quickly, so keep an eye on the beans to avoid burning them. Remove the skillet from the heat, cover, and let sit for about 5 minutes.
After 5 minutes, taste the beans. If still a bit firm, cover again and let rest for another 5 to 10 minutes. Tenderness will depend on the type of bean you’re using, so be sure to taste for texture.
Whisk the olive oil into the shallot mixture. When the beans are just tender, pour the shallot vinaigrette over the beans and toss to coat. Let the beans rest for another 5 to 10 minutes. Transfer the beans to a platter or bowl and drizzle on any shallot vinaigrette left in the bowl. Finish with the walnuts and black pepper before serving.