Preheat oven to 350 degrees F. Season chicken with salt and pepper, toss with 2 tablespoons oil, and bake in a cast-iron skillet until cooked through, about 30 minutes. Let cool, shred, and set aside. In a large skillet over medium heat, heat 1 1/2 teaspoons oil and cook tortillas in batches until soft, about 1 minute per side, adding oil as needed. Place 1/4 cup reserved chicken, 1 tablespoon cheese, and 1 teaspoon onion on each tortilla and roll into a cigar shape. Place in baking dishes and cover with remaining sour cream mixture and cheese. Bake until top is lightly browned, about 25 minutes. Garnish with remaining cilantro.