Preheat the oven to 400°F.
In a bowl, toss cubed chicken with garlic powder, onion powder, oregano, celery salt, paprika, black pepper, lemon zest, lemon juice, and olive oil.
Heat a skillet over medium and sauté chicken for a few minutes until it starts to cook through. Add onion and cilantro stems and cook for another 2–3 minutes, until the onion softens.
Lightly brush both sides of each tortilla with olive oil. Lay them on a baking sheet.
Spoon chicken mixture onto one half of each tortilla. Add a handful of shredded cheese. Fold closed and press gently.
Sprinkle a little more cheese on top (optional but encouraged).
Bake for 15–18 minutes, until the tortillas are golden and crispy and the cheese is melted and bubbling.
While tacos bake, blend the ranch ingredients (avocado, buttermilk, mayo or Greek yogurt, cilantro, dill, and a pinch of salt) until smooth.
Drizzle ranch over warm tacos or serve on the side for dipping. Done!