I was troubled since my kids would not eat bell peppers! Then, at a lunch menu tasting event held at their kindergarten, I learned from the cook that parboiling the bell peppers removes the bitterness. I made it right away and was amazed!
We overcame the "I-hate-peppers" phase!!

Be sure to add the salt to the hot water! Parboiling the bell peppers removes the bitterness. For 2 to 3 servings. Recipe by Nattsunaoko