If you are using homemade teriyaki sauce, prepare it first. If you are using store-bought, skip to the next step. To prepare the homemade teriyaki, whisk together all of the ingredients in a medium sauce pot. Turn the heat to medium-high and cook, whisking frequently, until the sauce is thick enough to coat the back of a spoon. Remove from heat.
Preheat oven to 425 degrees and line a large sheet pan with parchment paper. Lay the chicken breasts down the center of the sheet pan. Season with salt and pepper. Pour a little of the teriyaki sauce over the chicken breasts (about 1 tbsp of sauce per breast) and brush over both sides of the chicken. Be mindful to not contaminate the teriyaki sauce with the raw chicken. Bake the chicken for 10 minutes.
While the chicken is baking, combine the pineapple, peppers, and onion in a large mixing bowl. Add 4 tbsp of teriyaki sauce and toss to coat.
After the chicken has baked for 10 minutes, remove from the oven. Remove the chicken to a plate (wipe up the chicken juices a little) and spread out the pineapple, peppers, and onion across the sheet pan.
Nestle the chicken back onto the sheet pan. Try to make sure that everything is in an even layer and spread out.
Bake for another 15 minutes until the chicken is cooked through (should have an internal temp of 165) and the veggies are soft. Then turn the oven to broil (low) and broil for about 5-10 minutes until the pineapple and veggies are slightly soft and the teriyaki sauce on the chicken has caramelized.
Remove from the oven. This meal is fantastic when served over rice or quinoa (with a little extra teriyaki sauce) and then top with toasted sesame seeds, toasted coconut, or chopped fresh parsley (optional). Enjoy!