Combine the water and rice wine vinegar in a large bowl. Using a mandolin fitted with a fine julienne attachment, shave the russet potato directly into the water bath. Allow the potato to soak for a few minutes to remove excess starch.
Lift the potatoes directly from the water, leaving the starch behind in the bowl. Transfer to paper towels and dry thoroughly.
Heat neutral oil to 350°F. Fry the potato hay in small batches until deeply golden brown and crisp. Transfer to paper towels, season immediately with kosher salt, and spread onto a sheet tray to cool. For extra crunch, place the potato hay in a 170°F oven for 10 to 15 minutes to dry further.
Place a fine mesh strainer over a bowl. Grate the Persian cucumbers over the strainer, followed by the jalapeño. Allow the liquid to drain naturally until you have approximately 360 g of cucumber-jalapeño liquid.
Season the aguachile with lime juice and kosher salt. Taste and adjust as needed. The finished aguachile should taste bright, fresh, spicy, and lightly seasoned.
Add the cilantro, spinach, neutral oil, and a pinch of salt to a blender. Blend on high speed for approximately 5 minutes until the mixture becomes warm and vividly green.
Transfer the blended mixture to a piping bag or fine strainer setup and allow the oil to separate naturally from the solids. Strain and reserve the bright green cilantro oil.
Place sheets of parchment paper inside a tortilla press. Arrange slices of sushi-grade fish in an overlapping pattern. Press gently to create a thin, even layer of fish without tearing it.
Carefully transfer the pressed fish to chilled serving plates or bowls.
Spoon the cucumber aguachile around and over the fish. Drizzle with cilantro oil.
Scatter the pressed ceviche with sliced jalapeños, pickled red onions, and fresh cilantro.
Finish with a generous mound of crispy potato hay and season with flake salt.
Serve immediately while the fish is cold and the potato hay remains crisp.