How to Make It
Bring a large saucepan of salted water to a boil. In a medium saucepan, heat the vegetable oil. Add the red curry paste and cook over moderately high heat, stirring, until fragrant, about 30 seconds. Add the tomatoes and their juices and boil over moderately high heat until reduced to 3/4 cup, about 4 minutes. Add the chicken stock, coconut milk and brown sugar and simmer over moderate heat, stirring occasionally, until reduced to 1 1/2 cups, about 7 minutes. Remove from the heat and stir in the lime juice and fish sauce.
Line a plate with paper towels. Add the bok choy stems and leaves to the boiling water and cook until crisp-tender, about 1 minute. Using a slotted spoon, transfer the bok choy to the paper towels and pat dry.
Add the soba to the boiling water and cook, stirring occasionally, until al dente. Drain the soba and rinse in a colander under cold water. Let stand for 5 minutes, tossing occasionally, until dry.
Transfer the soba to a large bowl. Add the pickled ginger, sesame seeds, 1 cup of the red curry sauce and all but 1/2 cup of the bok choy leaves and toss well. Arrange the soba salad in shallow bowls and drizzle with the remaining sauce. Garnish with the cashews, bean sprouts, scallion and the remaining bok choy and serve.