Thinly slice the 1 large English cucumber thinly sliced into rounds, about 1/8 inch thick into rounds about 1/8 inch thick — a Mandoline makes this quick and uniform, but a sharp Knife works fine. Spread the slices in a single layer on a Parchment Paper-lined Baking Sheet, sprinkle lightly with ½ tsp sal kosher, and lay another Parchment Paper on top. Let them sit for 10 minutes to draw out excess moisture. Pat dry thoroughly before using.

While the cucumbers drain, make the spread. In a Mixing Bowl, beat the softened 8 oz queso crema ablandado a temperatura ambiente and 3 tbsp mayonesa together with a Hand Mixer (or a sturdy Spatula) until smooth and fluffy, about 1–2 minutes.

Add the fresh 1 tbsp eneldo fresco picado finamente, 1 tbsp cebollino fresco picado finamente, 1 tsp jugo de limón recién exprimido, ½ tsp polvo de ajo, ¼ tsp polvo de cebolla, ½ tsp sal kosher, and ½ tsp sal kosher. Mix until fully combined. Taste and adjust seasoning — it should be bright, herby, and well-seasoned. Set aside.

Lay all 16 bread slices out on a clean surface. If using 4 tbsp mantequilla sin sal ablandada, barrera de humedad opcional as a moisture barrier (recommended for make-ahead), spread a very thin layer of softened 4 tbsp mantequilla sin sal ablandada, barrera de humedad opcional on every slice first. This helps prevent the bread from getting soggy.

Spread a generous, even layer of the 8 oz queso crema ablandado a temperatura ambiente mixture (about 2 tablespoons) onto one side of each bread slice, going all the way to the edges.

Arrange the patted-dry 1 large English cucumber thinly sliced into rounds, about 1/8 inch thick slices in a single, slightly overlapping layer over the 8 oz queso crema ablandado a temperatura ambiente on 8 of the bread slices. Use enough slices to cover the bread completely from edge to edge.

Press the remaining 16 oz queso crema ablandado a temperatura ambiente-spread slices firmly on top, spread-side down, to form sandwiches. Press gently but firmly so everything adheres.

Using a sharp serrated Knife, trim the crusts off all four sides of each sandwich for a classic, neat presentation. Then cut each sandwich into 3 fingers (rectangles) or 4 triangles. Wipe the Knife blade between cuts for clean edges.

Arrange on a serving platter. Garnish with fresh 1 tbsp eneldo fresco picado finamente sprigs if desired. If not serving immediately, cover the platter tightly with Plastic Wrap and refrigerate for up to 4 hours. Serve chilled or at cool room temperature.