Pat the 1 lb solomillo de res, centro, sin pellejo de plata, atado en un cilindro uniforme si es necesario completely dry with paper towels and season all over with ½ tsp sal kosher and ½ tsp sal kosher. Heat 1 tbsp aceite de oliva in a Cast Iron Skillet over high heat until it just begins to smoke. Sear the 1 lb solomillo de res, centro, sin pellejo de plata, atado en un cilindro uniforme si es necesario on all sides — including the ends — for about 1 minute per side, until deeply browned all over. This should take about 4–5 minutes total. Transfer to a plate and let cool for 10 minutes.

Once the 1 lb solomillo de res, centro, sin pellejo de plata, atado en un cilindro uniforme si es necesario has cooled slightly, brush it all over with a thin, even layer of 1 tbsp mostaza Dijon. Place in the Refrigerator uncovered for at least 20 minutes (or up to 1 hour) to chill and set the 1 tbsp mostaza Dijon.

Pulse the 10 oz champiñones cremini finamente picados o procesados en un procesador de alimentos in a Food Processor until very finely chopped (or chop by hand). Melt 1 tbsp mantequilla sin sal in the same Cast Iron Skillet over medium-high heat. Add the 1 chalota pequeña finamente picada and cook for 1 minute, then add the 2 dientes de ajo picados and cook 30 seconds more. Add the 10 oz champiñones cremini finamente picados o procesados en un procesador de alimentos and 1 tsp hojas de tomillo fresco, season with ½ tsp sal kosher, and cook, stirring frequently, for 10–12 minutes until all the moisture has evaporated and the mixture is dark, dry, and paste-like. Spread onto a plate and refrigerate until completely cool, about 15 minutes.

Lay a large sheet of Plastic Wrap on your work surface. Arrange the 6 rebanadas prosciutto rebanadas delgadas, aproximadamente 3 oz en total slices in a single, overlapping layer to form a rectangle roughly 10x8 inches — wide enough to wrap the 1 lb solomillo de res, centro, sin pellejo de plata, atado en un cilindro uniforme si es necesario and long enough to cover it end to end. Spread the cooled duxelles evenly over the 6 rebanadas prosciutto rebanadas delgadas, aproximadamente 3 oz en total in a thin, uniform layer, leaving a 1-inch border on the long edges.

Place the chilled 1 lb solomillo de res, centro, sin pellejo de plata, atado en un cilindro uniforme si es necesario along the bottom edge of the 6 rebanadas prosciutto rebanadas delgadas, aproximadamente 3 oz en total. Using the Plastic Wrap to help, tightly roll the 6 rebanadas prosciutto rebanadas delgadas, aproximadamente 3 oz en total and duxelles around the 1 lb solomillo de res, centro, sin pellejo de plata, atado en un cilindro uniforme si es necesario, twisting the ends of the Plastic Wrap like a candy wrapper to form a tight, compact log. Refrigerate for at least 20 minutes (or up to 4 hours).

Preheat your Oven to 425°F. Line a Baking Sheet with Parchment Paper. On a lightly floured surface, unfold one sheet of thawed 1 paquete hojaldre congelado descongelado toda la noche en el refrigerador; una hoja utilizada and roll it out slightly to about 12x10 inches. Brush the entire surface with 1 huevo batido con 1 tbsp agua, para el barniz de huevo wash.

Unwrap the chilled 1 lb solomillo de res, centro, sin pellejo de plata, atado en un cilindro uniforme si es necesario log and place it in the center of the 1 paquete hojaldre congelado descongelado toda la noche en el refrigerador; una hoja utilizada. Roll the 1 paquete hojaldre congelado descongelado toda la noche en el refrigerador; una hoja utilizada tightly around the 1 lb solomillo de res, centro, sin pellejo de plata, atado en un cilindro uniforme si es necesario, pressing to seal the seam firmly. Tuck the ends under like a gift wrap and press to seal. Place seam-side down on the prepared Baking Sheet. Brush the entire surface generously with 1 huevo batido con 1 tbsp agua, para el barniz de huevo wash. Use a sharp knife to lightly score a crosshatch or diagonal pattern on top (do not cut through). Sprinkle with ½ tsp sal kosher if using. Refrigerate for 10 minutes before baking.

Bake at 425°F for 22–28 minutes, until the 1 paquete hojaldre congelado descongelado toda la noche en el refrigerador; una hoja utilizada is deep golden brown and an instant-read thermometer inserted into the center of the 1 lb solomillo de res, centro, sin pellejo de plata, atado en un cilindro uniforme si es necesario reads 125°F for medium-rare (the temperature will rise 5°F as it rests). For medium, pull at 130–132°F. Do not open the Oven door during the first 20 minutes.

Transfer the Wellington to a Cutting Board and let it rest, uncovered, for 8–10 minutes. Use a sharp Bread Knife to slice into two thick portions and serve immediately.