Char the poblano pepper over a gas flame or under the broiler until the skin is blistered and blackened.
Place the pepper in a bowl and cover with a lid for 5 minutes to steam, then peel off the skin, remove seeds, and dice.
Season the chicken breast with sea salt, black pepper, and garlic powder.
Heat avocado oil in a skillet over medium heat and cook the chicken until golden and cooked through, then shred with two forks.
In a small bowl, combine the shredded chicken with the diced poblano and half of the Greek yogurt.
In a separate bowl, whisk the remaining Greek yogurt into the tomatillo salsa until smooth.
Warm the tortillas slightly, fill with the chicken mixture, roll tightly, and place seam-side down in a baking dish.
Pour the creamy salsa over the tortillas, sprinkle with Monterey Jack cheese, and bake at 375°F for 10 minutes until bubbly.