For the curtido, place the vinegar in a large bowl and stir in the sugar. Add the chillies and then stir in the cabbage. Cover the bowl and leave to rest in the fridge for at least a day.
For the sweet potato and lentils, blitz the parsley, roast garlic and spring onion in a food processor until it forms a purée. Add the purée, along with all of the other ingredients, to a bowl and stir to combine. Season with salt and pepper and set aside.
For the chilli date purée, blitz the dates and chillies in a food processor. Gradually add some water until a paste is formed. The consistency you are looking for should be thin enough to squeeze out of a tube but thick enough to maintain its shape.
For the avocado mash, combine the avocado with the juice from the lemon and lime. Mash again until almost smooth. Season with salt and pepper and transfer to a small serving bowl. Cover with the olive oil to prevent discolouration.
To prepare the tortillas, cut small circles from the tortillas, you can usually get around two per tortilla. Heat a shallow amount of oil in a deep saucepan until very hot. (CAUTION: hot oil can be dangerous. Do not leave unattended.) Carefully add the tortilla circles to the hot oil and cook for about 45 seconds, or until crisp and golden brown. Transfer using a slotted spoon to a plate lined with kitchen paper.
Serve the curtido, sweet potato mixture, chilli date purée, avocado mash and duck confit in small bowls. Allow people to construct the tostadas using the fried tortillas and all of the ingredients.