In a non reactive bowl, stir halibut with onion, lime juice and vinegar to coat. Cover and refrigerate for 4 to 6 hours.
Discard halibut marinade and in a clean bowl combine halibut and bell pepper.
For dressing combine lime juice and rice vinegar and while whisking, add olive oil in a slow stream. Add lime zest, chives, cilantro, mint and chili flakes. Fold in halibut and bell pepper mixture and season, to taste, with salt and pepper. Spoon into martini glasses and top with radish sprouts.