Slice the 1 manojo rábanos tallos recortados, cortados finamente (aproximadamente 1/16 pulgada) as thinly as possible — about 1/16 inch thick. A Mandoline works best, but a sharp Knife is perfectly fine. Place them in a small Mixing Bowl of ice water for 5 minutes to make them extra crisp and curl slightly, then drain and pat dry with a paper towel.

Toast the 4 rebanadas pan de masa madre slices until deeply golden and crisp on the outside but still with a little chew inside. A Toaster Oven, Broiler, or dry Skillet over medium-high heat all work well. You want real color — pale toast won't hold up to the toppings.

While the toast is still warm, spread each slice generously with cold 4 cdas mantequilla sin sal estilo europeo fría, recién sacada del refrigerador — about 1 tablespoon per slice. Use the back of a spoon or a Knife to press it on thickly. The contrast of cold, creamy 4 cdas mantequilla sin sal estilo europeo fría, recién sacada del refrigerador against warm, crunchy toast is the whole point. Don't let it melt completely.

Arrange the sliced 1 manojo rábanos tallos recortados, cortados finamente (aproximadamente 1/16 pulgada) over the 4 cdas mantequilla sin sal estilo europeo fría, recién sacada del refrigerador in a single, overlapping layer, covering the toast edge to edge. If using 1 cdta jugo de limón recién exprimido, give the 1 manojo rábanos tallos recortados, cortados finamente (aproximadamente 1/16 pulgada) a very light squeeze now for a little brightness.

Finish with a generous pinch of 1 cdta sal marina en escamas como Maldon o fleur de sel over each slice. Add freshly ground 0.5 cdta pimienta negra recién molida and snipped 2 cdas cebollino fresco finamente picado if using. Serve immediately — this is best eaten the moment it's assembled.