With a very sharp knife, cut the halibut into thin broad slices. Place in a bowl, and toss with the lime juice and grapefruit juice. Let sit at room temperature for 15 minutes
Meanwhile, cut the grapefruit in half, at the equator, and using a grapefruit knife, cut out pieces of grapefruit. Slice each piece in half, the long way
When ready to serve ceviche, drain the liquid completely from the halibut, and discard liquid. Add grapefruit pieces to the fish, along with garlic, red chiles, green chiles, and mint. Toss gently. Divide among 6 plates, laying out the strips of halibut flatly on each plate. Season with coarse salt, sprinkle with hot sauce, and drizzle with extra-virgin olive oil. Serve immediately.