To prepare the tomatillo sauce, set the broiler to high. Place the onions, garlic, tomatillos and jalapeño on a sheet tray. Place it under the broiler and cook, turning occasionally, until nicely charred.

Transfer the vegetables to a blender and puree until smooth. Stir in the ground cumin, and season with salt and pepper. Set aside.

Heat a wide, heavy pan over medium-high heat, and add the grapeseed oil. Season the chicken with salt and pepper to taste, and sear in the hot pan, presentation-side down, until golden brown. Flip and repeat.

Pour the sauce and chicken stock into the pan with the chicken.

Bring the sauce to a boil, and reduce the heat to a simmer. Cover, and cook until the chicken is tender, about 40 minutes.

Stir in the cilantro, and adjust the seasoning with salt and pepper.

Transfer the chicken to a cutting board, and cut into thin strips. Return the chicken to the sauce.

To build the tacos, nestle the chicken and some of the sauce into the tortillas. Top with a sprinkling queso fresco and serve.