For the ceviche: Squeeze 2 ounces of the lime juice over the lobster and sprinkle with salt. Let sit to cook in this mixture for 10 minutes.
Add the scallions and continue to let the lobster sit until desired firmness is reached. Stir in the tomatoes, coconut cream, coconut milk, remaining lime juice, another pinch salt, the red pepper flakes and the sugar. Marinate for a few more minutes, or longer for a firmer texture.
For the tostonos: Meanwhile, heat the vegetable oil until hot in a heavy skillet. Fry the plantains until three-quarters of the way cooked through, then flatten into a tortilla shape while still hot using a wood pilon or roller. Re-fry until very crispy, then remove, toss with salt and cut in half or in quarters.
Serve the ceviche in a glass ramekin or in a coconut with the tostonos alongside.