Chop the fish into ½ in cubes or roughly chop the prawns. Cover and set aside in the fridge until a few minutes before you're ready to serve.
Segment one grapefruit, slicing the segments into quarters, making small grapefruit pieces. Dice the red onion, peel and finely chop the mango and avocado, dice the chili pepper, removing the seeds and membrane, roughly chop the parsley. Add to a mixing bowl and gently mix to combine.
Juice the second grapefruit and add the juice to a small mixing bowl or liquid measure along with the olive oil, sea salt and pepper. Whisk together to combine. Remove the prepared seafood from the fridge. Pour the liquid over the seafood and stir well to combine and coat. Let sit one minute, then stir the seafood mixture again and let sit an additional 2 minutes allowing the acid in the grapefruit juice to ‘cook’ the seafood. You will notice the prawns turning pink and fish turning from opaque to white as well.
Add the seafood to the larger mixing bowl with the mango mixture and gently mix to combine the seafood into the mixture and coat everything well with the juice. Cover and let sit in the fridge, while you prepare the tostadas.
To prepare the Tostadas, in a small frying pan add the neutral oil and place over medium high heat. Heat the oil for 2 to 3 minutes until hot, then reduce the heat to medium. Slice the corn tortillas into quarters.
Place four quarters into the pan at a time, they will sizzle and start to curl. Cook for 15 to 20 seconds until golden and crisp, turn over and cook an additional 15 seconds, to crisp up both sides. Remove from the pan with tongs and place in a paper towel lined serving basket. Repeat with the remaining corn tortillas.
Remove ceviche from the fridge, spoon into a serving bowl and drizzle with a little additional olive oil, sprinkle with salt and pepper to taste. Serve with the hot and fresh tostadas.
Ceviche does not keep and should be enjoyed the day it’s made. The tostadas are also best enjoyed the day they are made and soften when stored.