Cut each poblano in half lengthwise and remove core and seeds. Place peppers on baking sheet and brush with 1 tsp oil. Roast until tender and darkened in places, 13 to 15 minutes.
Meanwhile, in large skillet over medium, heat 2 tsp oil. Add turkey and onion; cook, breaking up turkey into smaller pieces, until turkey is no longer pink and onion is translucent, 5 to 8 minutes. Add garlic; heat for 1 minute. Stir in oregano, cumin, and salt; heat for 1 minute. Place marinara sauce and chipotle pepper in pan and heat for 5 minutes, stirring occasionally. Stir in masa harina, if using.
Divide turkey mixture among baked poblano halves and top with cheese. Bake until cheese has melted. Serve topped with diced avocado, sour cream, and cilantro.
Per serving: 454 calories; 28 g protein; 30 g total fat (11 g sat. fat); 19 g total carbohydrates (7 g sugars, 6 g fiber); 677 mg sodium