Rinse the chicken breasts and pat dry with a paper towel.
Cut the chicken breasts into 1-inch cubes.
Combine the soy sauce and the ginger in a shallow glass dish, then add the chicken chunks and marinate for about 45 minutes, stirring occasionally.
While the chicken marinates, pour the broth and the pineapple juice into a small bowl, then add the corn starch, stirring to dissolve.
Toast the sesame seeds.
Heat the oil over high heat in a non-stick wok or large skillet.
Add the chicken and stir-fry for 2 minutes. Remove the chicken from the skillet, leaving any juices in the pan.
Stir-fry the peppers and the onions for 1 minute.
Add the pineapple to the skillet and return the chicken to the skillet. Stir-fry for 2 or 3 more minutes.
Stir the pineapple juice mixture into the skillet and boil about 1 minute until the sauce is thick and smooth.
Serve with sesame seeds on top.