Wash and clean the peppers well, taking care to remove the seeds and stalk, and cut them into pieces. Skin the onion and also cut it into pieces. Put the peppers and onions in an immersion blender then add the grated cheese and a little oil.
Blend the ingredients and if the pesto seems too thick add a little cold water instead of oil to make pepper pesto lighter. Add salt and mix again. Pour into a bowl and set aside.
Meanwhile, cook the spaghetti alla chitarra for 3 minutes, drain well and pour into the bowl with the pesto.
Mix well, serve immediately and enjoy