Place the poblano pepper under a broiler for 5-7 minutes, turning occasionally until the skin is charred and blistered.
Place the charred pepper in a bowl and cover with plastic wrap for 10 minutes to steam, then peel off the skin, remove seeds, and slice into thin strips.
Season the chicken breast with sea salt, black pepper, and garlic powder, then pan-sear in olive oil over medium heat until cooked through and shred using two forks.
In a small mixing bowl, combine the shredded chicken with half of the roasted poblano strips and 2 tablespoons of the tomatillo salsa.
Briefly warm the corn tortillas in a dry pan or microwave until pliable, then fill each with the chicken mixture and roll tightly.
Arrange the rolled tortillas seam-side down in a small baking dish, pour the remaining tomatillo salsa over the top, and sprinkle with shredded cheddar cheese.
Bake at 375°F for 15 minutes until the cheese is melted and the sauce is bubbling.
Serve topped with a dollop of Greek yogurt, the remaining poblano strips, and fresh cilantro.