How to Make It
In a small food processor, puree the sour cream with the garlic, cilantro and the juice of 1 lime. Season with salt.
In a shallow bowl, whisk the remaining lime juice with the cumin, 2 tablespoons of olive oil and 1 teaspoon of salt. Add the chicken and turn to coat with the marinade. Let stand at room temperature for 10 minutes.
Meanwhile, light a grill or preheat a grill pan and brush the grates with oil. Grill the scallions over high heat, turning frequently, until wilted and lightly charred, about 4 minutes. Transfer to a plate.
Remove the chicken from the marinade and grill, turning once, until lightly charred on the outside and cooked through, about 8 minutes. Transfer the chicken to a cutting board and let cool for 5 minutes, then thinly slice.
Grill the tortillas in a single layer, turning once, until soft and pliable, about 30 seconds. Spread the tortillas with crema and top with the scallions and chicken. Serve right away.