In a bowl, whisk marinade ingredients (all except chicken and halved limes) together.
Marinate thighs in the refrigerator for 1 hour or more.
Preheat barbecue to medium-high.
Remove chicken from the marinade, scraping off excess marinade.
Grill 7 - 8 minutes per side. Reserve.
Toss sliced onions with olive oil and grill 2 minutes per side.
Divide chicken among 4 plates and garnish each plate with half a lime, cilantro ,and chilli flakes.