Grilled Chicken Tacos

Name: Grilled Chicken Tacos
Ingredients: 1 medium onion, cut into wedges, keeping root intact; 2 garlic cloves, finely chopped; 1 pound skinless, boneless chicken thighs; 1 tablespoon cumin seeds, coarsely crushed; 1 tablespoon vegetable oil; 1 teaspoon kosher salt; 1 /2 teaspoon freshly ground black pepper; 8 corn tortillas, warmed (for serving); 2 avocados, sliced (for serving); Charred Tomatillo Salsa Verde (click for recipe; for serving); Cilantro sprigs, sliced radishes, and lime wedges (for serving)