Preheat oven to 375 degrees Fahrenheit. Mix the natto with complimentary sauce or with a little soy sauce and yellow mustard. Trim off the ends of chili pepper. Remove the seeds using a chopstick or small spoon. Fill chili pepper with natto. Push in with a small spoon. If you run out of natto, just trim the chili by cutting off at the point where no more natto filling is present. Place the chili rolls on a baking sheet. Bake 15 to 20 minutes. Cut into smaller segments (optional) or serve with knife and fork.