How to Make It
Light a grill or preheat a grill pan. Brush the eggplant with olive oil and season with salt. Grill over moderate heat, turning once, until tender and lightly charred, 6 minutes. Transfer the eggplant to a plate and let cool completely, then cut into 3/4-inch pieces.
In a small skillet, toast the pine nuts over moderate heat, tossing, until golden, about 4 minutes; transfer to a plate and let cool.
In a food processor, combine the pine nuts, basil and garlic and pulse until finely chopped. With the machine on, add the 1 cup of olive oil in a steady stream. Add the soy sauce and pulse to blend. Season the pesto with salt.
In a large saucepan of salted boiling water, cook the soba noodles until al dente, about 4 minutes. Rinse the noodles under cold water, then drain and pat dry.
In a large bowl, toss the soba noodles with the pesto until evenly coated. Add the grilled eggplant, tomatoes, radishes, cucumber and 3/4 cup of the mint leaves and toss well. Season the soba noodle salad with salt. Sprinkle the remaining 1/4 cup of mint leaves on top and serve.