Place the beef tenderloin in the freezer for 20–30 minutes until very firm but not frozen solid; this makes slicing thinly much easier. While it chills, remove any silver skin or large pieces of fat with a sharp knife, leaving a smooth cylinder of meat.

Slice thinly

Using a very sharp chef's knife, slice the chilled beef as thin as possible (aim for 1–2 mm). If you have one, you may use a meat slicer at a thin setting or place slices between plastic wrap and gently pound with a heavy rolling pin to even them out. Arrange slices in a single overlapping layer on chilled plates or a serving platter.

Season beef

Lightly season the beef with about half of the fine sea salt and half of the black pepper. Let rest at cool room temperature for 2–3 minutes so the flavors begin to open.

Add arugula and onions

Toss the arugula with a very light drizzle (about 1 tsp) of extra-virgin olive oil and a pinch of salt to coat. Scatter the dressed arugula evenly over or beside the beef slices. Sprinkle the finely chopped red onion evenly over the beef.

Finish garnishes

Evenly distribute the drained capers over the plated beef and shave the Pecorino Romano over the top. Finish with the remaining pinch of salt if needed and the remaining black pepper. Drizzle the reserved tablespoon of olive oil lightly over the entire dish for gloss and mouthfeel.

Mustard Aioli

Make the aioli

In a small bowl, combine mayonnaise, Dijon mustard, whole-grain mustard, lemon juice, minced garlic, olive oil, salt, and pepper. Whisk until smooth and emulsified. Taste and adjust seasoning — add a few drops more lemon for brightness or a little more Dijon for bite.

Plate with sauce

Using a spoon or small squeeze bottle, dot or lightly zigzag the mustard aioli over the plated beef carpaccio. Aim for small amounts so the beef remains the star; about 1–1.5 tablespoons of aioli per serving total. Alternatively, serve the aioli on the side.