How to Make It
Prepare the vinaigrette and noodles In a small skillet, simmer the orange juice over moderate heat until syrupy, about 15 minutes. Scrape into a medium bowl and whisk in the lime juice, soy sauce, sesame oil, vinegar and sugar; season the vinaigrette with salt.
Meanwhile, in a large pot of salted boiling water, cook the soba noodles until al dente, about 4 minutes. Drain and rinse under cold water; shake off any water and blot dry. Transfer the noodles to a large bowl. Add the scallions and sesame seeds and toss with three-fourths of the vinaigrette. Add the tofu to the remaining vinaigrette.
Cook the vegetables In a large skillet or wok, heat the vegetable oil. Add the onion and stir-fry over high heat until starting to brown, about 3 minutes. Add the broccoli, cauliflower, fennel, carrot and ginger and stir-fry until the vegetables are tender, about 5 minutes. Add the vinegar and chard leaves and cook until wilted, about 2 minutes. Add the vegetables to the bowl with the soba, top with the tofu and serve with Sriracha.