Electric Pressure Cooker - Turn your Instant Pot to Saute and add the olive oil, onion, bell peppers and salt. Saute for 3-4 minutes or until the vegetables are softened. Add the marinara, pesto and chicken. Cook thawed chicken on high pressure for 12 minutes. (Cook frozen chicken on high pressure for 20 minutes.) Turn the pressure valve to “Vent” to release all of the pressure.
Remove the chicken breasts from the pot and place on a plate or cutting board. Shred the chicken using two forks.
Remove ⅔ cup of liquid from the pot, being careful to leave the vegetables. (Reserve the liquid for cooking rice or making soup.) Add the mushrooms to the vegetables and set to saute for 2-3 minutes until mushrooms are slightly softened. Return the shredded chicken to pot and stir until well combined.
Crock Pot - Add the chicken breasts to a sprayed or lined Crock Pot. Drizzle with olive oil and sprinkle with salt. Top with the bell peppers, onion, marinara and pesto. Add an additional ½ c. marinara.
Cook on low for 4-5 hours. With 30 minutes of cooking time left, shred the chicken using two forks and add the mushrooms.
(If using frozen chicken in the Crock Pot, cook on low for 5-6 hours.)