Cook the noodles in a generous amount of boiling water until tender, about 5 minutes. Drain well, cool in cold water, drain well again and place in a large bowl. Add the sesame oil to the noodles and toss to coat. Cover and refrigerate noodles until needed. (Noodles can be prepared several hours before needed.)
Make dressing by combining stock, soy sauce, vinegar, sugar and chili flakes in a medium bowl. Set the carrot, bell pepper, cucumber, radishes and green onion in the bowl but do not toss. Cover vegetables and dressing until needed. (Vegetables can be prepared to this point several hours before needed.)
To finish salad, add the noodles and sliced lettuce to the dressing/vegetable mixture, toss to combine and serve.
Note: Soba noodles are available in our Asian foods aisle.