Combine grapefruit juice, lime juice, lime zest, sesame oil, red chili, grated ginger and finely chopped cilantro in a bowl. Season with sea salt and black pepper according to taste.
Cut the cod into equally sized cubes and add to the grapefruit marinade. Place in the fridge and let it rest for at least 10 minutes, but preferably longer.
When ready to serve cut the cucumber, separate the grapefruit, shave the fennel and peel the shrimps.
Combine the cod with cucumber, grapefruit and half of the fennel.
Divide between two bowls (save the leftover liquid from the marinate) and top with the remaining fennel and add the shrimps on top. Pour the leftover marinade over the ceviche and then top with sesame seeds and cilantro.