Mix 1 cup plain whole-milk yogurt, 1 tsp. freshly ground pepper, 1 tsp. ground turmeric, and 1 Tbsp. Diamond Crystal or 1¾ tsp. Morton kosher salt in a large bowl to combine. Add 1 medium onion, thinly sliced, and 2¼ lb. skin-on, bone-in chicken thighs (4–6) and toss to coat, breaking up onion. Cover and chill at least 12 hours and up to 3 days.
Prepare a grill for medium-high indirect heat (for a charcoal grill, bank coals on one side of grill; for a gas grill, leave one or two burners off). Lightly oil grate with vegetable oil. Place chicken, skin side down, over direct heat and grill, turning every minute, until lightly browned, 7–10 minutes. Move chicken over indirect heat, cover, and grill, turning every 5 minutes, until an instant-read thermometer inserted into thickest part of a thigh registers 165°, 20–25 minutes.
Just before chicken is done, heat 1 lime, zest removed in wide strips, 2 Tbsp. honey, and 1 tsp. mild red pepper flakes (such as Aleppo-style or Urfa biber) in a small saucepan on grill over indirect heat, swirling, until honey is loose and warmed through, about 1 minute. Brush chicken with glaze on all sides. Transfer to a platter and sprinkle with flaky sea salt. Let rest 5–10 minutes.
Meanwhile, place 1 cup coarsely chopped mixed tender herbs (such as mint, dill, basil, cilantro, parsley, and/or tarragon) in a small bowl; season with kosher salt. Squeeze 1 lime half over and toss to coat.
Scatter herbs over chicken; tuck 1 small bunch red radishes, trimmed, halved, quartered if large, around. Cut remaining 1 lime half into wedges and serve alongside.