Preheat the oven to 375°F (190°C).
Thinly slice the onion and arrange the slices in an even layer on the bottom of a casserole or baking dish.
Pat the chicken filets dry and season both sides with the salt, pepper, and garlic powder.
Lay the seasoned chicken filets on top of the bed of onions in a single layer without overlapping.
Spread the BBQ sauce evenly over the top of each chicken filet.
Sprinkle the shredded cheddar cheese evenly over the sauced chicken.
Bake in the preheated oven for 30–40 minutes, or until the chicken reaches 165°F (74°C) and is cooked through.
Remove from the oven and let rest a few minutes, then top with diced green onion and chopped cilantro before serving.