1 medium cucumber—peeled, seeded and sliced crosswise 1/4 inch thick
1 cup mung bean sprouts
2 scallions, thinly sliced
In a heatproof medium bowl, pour the boiling water over the ancho. Cover and let stand until softened, about 20 minutes.
Meanwhile, in a large pot of boiling water, cook the udon, stirring often, until just tender, about 8 minutes. Drain the udon in a colander and rinse well with cold water. Let drain, lifting the noodles occasionally to keep them separate.
Remove and discard the stem and seeds from the ancho and finely chop the chile. Transfer the ancho to a blender, add the orange juice, mirin and orange zest and blend to a smooth sauce. Add the sesame oil and season the vinaigrette with salt.
In a large bowl, toss the udon with the cucumber, bean sprouts and scallions. Add the orange dressing, toss well and serve immediately.