Charring tomatoes in a cast-iron skillet yields smoky results.
- Heat a large cast-iron skillet over high heat until very hot. Place tomatoes in the skillet and turn occasionally with tongs until charred on all sides, about 10 minutes. Transfer to a plate to cool slightly. Cut in half crosswise; squeeze to discard seeds. Chop the remaining pulp and skins; set aside.
- Add 1 teaspoon of the oil to the pan and heat over high heat until the oil is very hot. Add chicken and season with salt and pepper. Cook, stirring occasionally, until the chicken is browned on all sides and no longer pink in the center, about 5 minutes. Transfer to a plate and set aside.
- Reduce the heat to medium and add the remaining 1 teaspoon oil. Add onions and cook, stirring, until softened, about 5 minutes. Add garlic and jalapeños and cook, stirring, for 1 minute more. Add lime juice and the reserved chicken and tomatoes. Bring to a simmer and stir in cilantro and scallions. Season to taste with salt and pepper. Cover to keep warm.
- Spoon filling into warm tortillas, roll up and serve with sour cream and lime wedges.