2) Chop the spinach and add it to a medium bowl with the remaining tostada ingredients, except the extra-virgin olive oil. The mixture should be very well combined and moist, but not wet.
3) Line a baking sheet with greaseproof paper. Divide the mixture into 12 equal portions and form into flat patties, about 7.5cm in diameter and 0.5cm thick. Arrange the patties on the baking sheet and press down until very firm. This step can be done ahead, but if so refrigerate the patties until required.
4) In a large nonstick saute pan over medium-high heat, heat the oil. When hot, add the spinach tostadas to the pan, cook about 2 minutes, gently turn and cook the other side. They will be slightly browned. Put them on a paper towel lined baking sheet, sprinkle with a pinch of salt and hold in a warm oven.
1) In a small bowl combine the sour cream and lime zest and set aside.
2) Heat the remaining oil left in the pan the spinach tostadas were cooked in over a medium-high heat. When the oil is hot, add the shallot and cook until translucent.
3) Add the tomatoes, potatoes and capers, stir to combine, then add the prawns and stock and cook until the prawns turn pink. Stir in the pepper and the parsley and remove from the heat.
4) Remove the spinach tostadas from the oven, and transfer to a serving platter. Equally portion the prawn mixture on top of each tostada, sprinkle with feta, top with a small dollop of the sour cream mixture and serve immediately.