Brush the tortillas lightly with olive oil on both sides, cut discs out of the tortilla, fold them gently in halve paying attention not to break them and hang them over an oven rack. Bake in a moderate oven until tacos are crispy. Allow to cool completely before filling. Chop the prawns and avocado roughly, marinade with ½ of the lemon juice, season with salt and pepper. Mix with the mayo and ketchup, smoked paprika and chopped parsley. Fill up the taco shells with salad leaves, top up with the avocado-shrimp mix.