Place chicken in a sealable bowl or container. Add fajita marinade. Toss to fully cover chicken (Add more fajita marinade, if you like. 1 tablespoon seemed a bit sparse to us. - James)

Place container in refrigerator and marinate overnight.

Remove chicken from the refrigerator. Grill or bake until fully cooked to an internal temperature of at least 165°F.

Remove from grill and let cool to the touch.

Dice chicken into 1/4-inch cubes. Set aside.

Place oil in a skillet. Place over medium heat and heat until hot and shimmering.

Add yellow onions and sauté until translucent. Remove from heat and place in a mixing bowl.

Add chicken and barbecue sauce. Mix well.

Cover and place in refrigerator if not using immediately.

Preheat oven to 375°F.

Place nacho chips on a foil-lined baking sheet.

If chicken filling has cooled, place in a microwave-safe container and heat until just warm. Place a generous amount of chicken on each chip.

Sprinkle each with cheese.

Place in preheated oven and bake for 6 to 8 minutes, until cheese is melted and starting to bubble.

Arrange cooked tostados on a large platter.

Serve hot with ramekins or bowls of sour cream, pico de gallo and guacamole, to dress and/or dip. Garnish with cilantro.