1. Heat a large cast-iron skillet over medium-high heat. Add the cumin seeds and toast, stirring occasionally about 1 minute; transfer to a bowl. Add the garlic and chiles to the skillet. Cook, turning occasionally, until soft and blackened in spots, about 5 minutes. Transfer to a food processor an let cool. Add the cumin seeds, vegetable oil, 1/4 lime juice and the brown sugar and puree until smooth. Season with 1 1/2 teaspoons salt and pepper. Transfer to a resealable plastic bag and add the steak, shaking well to coat.
Refrigerate 1 to 4 hours.
2. Soak the onion in a bowl of cold water. Meanwhile wipe out the skillet and heat over high heat. Cook the tomatillos, turning until well charred, about 7 minutes. Transfer to a plate and let cool. Remove the charred husks and roughly chop the flesh. Drain the onion. Wipe out the food processor; add the tomatillos, onion, cilantro and remaining lime juice and puree until smooth. Season with 3/4 teaspoon salt and pepper; set aside.
3. Meanwhile, bring the steak to room temperature, about 30 minutes.
Preheat the grill to high. Remove the steak from the marinade and shake off any excess.
Lightly oil the grill then grill the steak until charred on the bottom, about 5 minutes.
Flip and cook until charred on the other side, 1 to 3 minutes/
Transfer to a cutting board and let rest 10 minutes so they juices wont come out of the meat.
4. Warm the tortillas on the grill. Thinly slice the steak against the grain. Double up the tortillas and fill with the steak and salsa. Top with cheese and radishes.