Adjust rack to middle position and preheat oven to 375°F.
In a medium pot, combine the ingredients for the poaching liquid. Bring to a boil.
Add the shrimp and cook for 3-4 minutes, until cooked through. Drain and let cool completely (TIP: Place bowl of shrimp into a large bowl filled with ice to cool quickly and safely. This step can also be done a day ahead of time).
Cut shrimp into small chunks. Place into a medium-sized bowl. Add the remaining ingredients for the ceviche (add half the jalapeno if you don't love spicy, then add more as you'd like). Season to taste with Kosher salt. Let sit for 10-15 minutes, stirring occasionally.
Lay tortillas out evenly, making sure they do not overlap, on a parchment-lined sheet pan. Spray to coat with avocado oil (if you don’t have a spray, brush on a thin layer of avocado oil). Cook for 5-6 minutes per side, until crisped through. Let cool.
Meanwhile, in a medium bowl, mash the avocado with juice from half a lime (double this amount if doubling the recipe). Season with Kosher salt and freshly ground black pepper, to taste.
To assemble, spread as much of the mashed avocado as you’d like into each tostada. Top with shrimp ceviche. Serve and enjoy!