Place a pot of water on the fire to boil the noodles. Boil the water, pour in the salt and place the noodles. Cook for 7 minutes. While the noodles are boiling prepare the vegetables. Slice 3/4 of the scallion bundle into 2.5-inch lengths. Chop the remaining onion finely to serve.
Peel the seeds and insides of the peppers. Slice the bell peppers into bars and the chili into thin circles. Press or finely chop the peeled garlic.
Pour oil into a pan and heat it. Add the vegetables and cook for about 2 minutes, stirring constantly. Pour in the vinegar, soy sauce and sesame paste.
Discard the noodles in a colander and rinse with cold water. Place them in a pan and heat through with the vegetables and sauce. Transfer the buckwheat soba to a serving plate and sprinkle with the sesame seeds and scallion.