Season the filet on all sides with Jacobsen Salt Co. X SRF Garlic and Black Pepper Infused Salt. Place a serving platter into the refrigerator to chill. Keep the platter chilled until ready to serve.
Heat a small skillet over medium-high heat. Add the oil, and the filet on the rounded side. Cook, rolling the steak every 20 seconds, until the outside is seared all around, about 2 minutes. Do not sear the ends.
Remove the filet from the pan, and set aside to cool for a few minutes before wrapping tightly in plastic wrap. Place the wrapped steak into the freezer to chill for 2 hours. This will make it easier to slice very thin.
Remove the filet from the freezer, unwrap, and place the seared side on a cutting board. On another surface, prepare a sheet of plastic wrap that is slightly larger than the serving platter. With a sharp knife, slice the filet into 1/8-inch-thick rounds, or as thinly as possible. Place the filet slices onto the prepared plastic wrap in a single layer. Top with another piece of plastic wrap, place on a stable cutting board or workspace, then gently pound with the flat surface of the bottom of a skillet, or roll out using a rolling pin. The filet should be as thin as possible without tearing. Leave the filet in the two layers of plastic wrap and keep refrigerated until ready to serve. This can be done a few hours ahead, but should be enjoyed the same day.
When ready to serve, remove one layer of the plastic wrap, then invert the sheet of filet pieces onto the chilled serving platter. Carefully remove and discard the other piece of plastic wrap. In a small bowl, add the watercress greens and some of the vinaigrette, and toss to combine. Arrange the dressed greens over the filet. Season with flaky salt, pepper, and top with shaved Pecorino. Drizzle with additional vinaigrette as desired, and serve immediately. Enjoy!
Make the horseradish vinaigrette; In a small bowl, add the oil, vinegar, mustard and horseradish and whisk well to combine. The vinaigrette should thicken slightly and be a uniform light-yellow color when the oil is emulsified.