Add the chicken, salsa and fajita seasoning to your slow cooker and cook on low for 6 hours (2-3 hours on high).
When done cooking, using two forks, pull apart the chicken until it is well shredded. I often do this in the slow cooker, but you can also remove the chicken and separate on a cutting board or another bowl.
Once the chicken is shredded, add in the sliced peppers and onions and cook on high for another 30-45 minutes. You just want to make sure the peppers are tender, but not soggy.
While the peppers are cooking, heat up the refried beans in a small saucepan on the oven (low for 10 minutes) or you can even heat it up in the microwave in a microwave safe dish.
You can also prepare all your toppings at this point, cutting up tomatoes, avocados, etc.
Once the chicken, peppers and onions are ready, spread a thin layer of refried beans on tostado shells and then top with the chicken mixture.
Add all your favorite toppings and enjoy!