1. Blanch the Canadian geoduck in boiling water for 5 – 10 seconds and then drench in an ice bath. Over a bowl, process and clean the geoduck by removing the shell, skin, and gut ball. Keep the clam nectar and set aside for the ceviche.
2. For the avocado puree: add the cilantro, lime juice, avocados, olive oil, serrano pepper, and salt to a blender. Blend until creamy. Taste for salt and add more if necessary.
3. For the ceviche: Drain the clam nectar with a paper towel and a strainer to extract the sand. In a large bowl, add the shallots, garlic, serrano pepper, lime juice, tangerine juice, and lemon juice. Rub the basil and mint together in your hands to bring out the natural flavors and add to the bowl. Combine the ceviche base with the clam nectar.
4. Clean the Canadian geoduck siphon by slicing it in half and giving it a quick rise to get rid of any sand. Thinly slice the geoduck siphon.
5. Marinate the slices of geoduck by slowly draining the liquid from the ceviche base on top. Finish lightly with olive oil. 6. Building your tostadas: Add a spoonful of avocado puree and geoduck ceviche to your tostadas. Top with cucumber, radish, chilli, and mint. Finish with salt and enjoy!