1 tablespoon vegetable oil
1 shallot, very thinly sliced
2-inch piece fresh ginger, peeled and cut into thin slivers
3 small carrots, cut into matchsticks
1 large watermelon radish, peeled and cut into matchsticks (or 4 to 5 red radishes, trimmed and quartered lengthwise)
2 tablespoons tamari or other soy sauce
1 tablespoon mirin (sweet Japanese rice wine)
4 packed cups spinach leaves (4 to 5 ounces)
1 1/2 cups cubed tofu (optional)
1 cup fresh or frozen peas
1 teaspoon rice vinegar, or to taste
8 ounces soba noodles (preferably 100% whole buckwheat), cooked until just tender, drained and rinsed under cold water
1 tablespoon toasted sesame seeds, for garnish
2 sliced scallions, for garnish
1. Heat a 10-inch skillet with a tight-fitting lid over medium heat for 30 seconds. Add the vegetable oil, shallot and ginger and cook, stirring, until the shallot is touched with brown, about 3 minutes.
2. Add the carrots and radish, stir, then pour in the soy sauce and mirin. Cover, reduce the heat to medium-low, and simmer until the vegetables are crisp-tender, about 3 minutes. Add the spinach, tofu if using, and peas. Cover, and simmer until the spinach is wilted and the tofu and peas are heated through, 4 to 5 more minutes. Stir well and season with the rice vinegar.
3. Meanwhile, rinse the soba with hot tap water until warm, drain well, and divide between shallow bowls. Spoon the braised vegetables and their sauce over the noodles and serve garnished with sesame seeds and scallion.