Rinse and clean sugar snap peas, spring onions and celery. Cut spring onions and celery into 4 cm long pieces. Make noodles in boiling hot salted water for 3 min. Add sugar snap peas, spring onions and celery, bring to boil repeatedly and do another 3 min over medium heat. Drain, reserving cooking water, drain and mix with 2 tbsp oil.
Tear green leaf lettuce, rinse and toss dry. Pluck leaves into bite-sized pieces. Rinse basil, dry and chop. Stir juice of one lemon with 3 tablespoons salt, pasta cooking water, mustard, pepper and remaining oil. Combine salt, pasta, vegetables, basil and sauce.