Stir water and salt together in a large bowl, submerge chicken breasts, then let sit at room temperature for 15 minutes.
Remove the breasts from the brine, then rinse and pat dry.
Pre-heat grill at medium high (400 degrees F) for 10 minutes.
Once hot, carefully oil the grates, then place chicken on the grill and grill for 5-8 minutes.
Flip and grill the other side of the chicken for an additional 5-8 minutes or until an internal temperature of 165 degrees F is reached. In the last minute of cook time, brush chicken with a thin layer of bbq sauce.
Remove chicken from the grill, then let rest for 5 minutes before cubing and tossing with more BBQ sauce.
Creamy Avocado Slaw
Mix together coleslaw, cabbage, green onions, and cilantro in a large bowl, then set aside.
Place avocado, greek yogurt, lime, garlic, and olive oil into a food processor, then process until smooth.
Toss the sauce with the slaw mixture, then season with salt and pepper, to taste.
Assembly
Fill tortillas with chicken, then top with slaw and crumbled cotija/feta and enjoy.