Steak Tacos with Tomatillo Salsa

Name: Steak Tacos with Tomatillo Salsa
Ingredients: 1 1/2 teaspoons cumin seeds; 2 cloves garlic; 4 serrano chile peppers, halved (remove seeds for less heat); 3 tablespoons vegetable oil, plus more for the grill; Juice of 4 limes (about 1/3 cup); 2 tablespoons packed light brown sugar; Kosher salt and freshly ground pepper; 2 pounds skirt steak, cut in half; 1/2 small white onion, diced; 3 tomatillos (in their husks); 3/4 cup fresh cilantro; 16 corn tortillas; Grated Cotija cheese and/or thinly sliced radishes, for topping