Add the spinach to a colander, and press down to remove as much water as possible. Transfer the spinach to a double layer of paper towels and press and squeeze to remove any extra water. Chop the spinach and add it to a medium bowl with the remaining ingredients, except the extra-virgin olive oil. The mixture should be very well combined and moist, but not wet.
Line a baking sheet with parchment paper. Divide the mixture into 12 equal portions and form into flat patties, about 3 inches in diameter and 1/4-inch thick. Arrange the patties on the baking sheet and press down until very firm. This step can be done ahead, but be sure to refrigerate the patties.
In a large nonstick saute pan over medium-high heat, heat the oil. When hot, add the spinach tostadas into 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.
Topping: In a small bowl combine the sour cream and lime zest and set aside, set aside.
In the same pan the spinach tostadas were cooked in, heat the remaining oil left in pan, over medium-high heat. When the oil is hot, add in the shallot and cook until translucent. Add in the tomatoes, potatoes and capers, stir to combine, add in the shrimp and stock and cook until shrimp turn pink. Stir in the pepper and the parsley and remove from the heat.
Remove the spinach tostadas from the oven, and transfer to a serving platter. Equally portion the shrimp mixture on each tostada, sprinkle with feta, top with a small dollop of the sour cream mixture and serve immediately.