Place the beef in the freezer for 20–30 minutes to firm it slightly; this makes hand-cutting easier. Meanwhile, line a tray with parchment. Trim any silver skin or excess fat from the beef.

Hand-cut the beef

Using a very sharp chef's knife, cut the beef into thin slices, then into fine 1/8–1/4-inch dice. Return the diced beef to the chilled bowl to keep it cold.

Prepare shallot and pickles

Finely mince the shallot, chop the rinsed capers, and finely dice the cornichons. Place them together in a small bowl.

Season the tartare

Add the minced shallot, capers, cornichons, Dijon mustard, 1 tablespoon extra-virgin olive oil, Worcestershire, lemon juice, 1/2 tsp salt and 1/2 tsp pepper to the diced beef. Gently fold with a rubber spatula or spoon until combined. Taste and adjust seasoning gently. Keep chilled until assembly.

Chimichurri

Make the chimichurri

Finely chop parsley, cilantro and oregano (or pulse briefly in a food processor). Mince the garlic. Combine herbs and garlic in a bowl with 1 tablespoon red wine vinegar, 3 tablespoons olive oil, red pepper flakes, 1/4 tsp salt and 1/4 tsp black pepper. Stir well and taste; adjust acidity or salt as needed. Let sit 10–15 minutes for flavors to marry.

Assembly & Garnish

Prep shells

If shells are store-bought crisp tacos or mini tostadas, lightly brush both sides with 1 tsp olive oil and toast in a 350°F (175°C) oven 4–6 minutes until warm and crisp. If making from soft tortillas, fry or bake until crisp. Sprinkle lightly with smoked paprika and a pinch of sea salt while still warm.

Portion the tartare

Using a small spoon or scoop, portion about 1.5 tablespoons of the beef tartare into each mini shell (yields ~12 tacos). Slightly mound the tartare in the center so it sits well in the shell.

Add chimichurri and garnish

Spoon about 1/2 teaspoon of chimichurri over each portion of tartare. Top with a few microgreens or frisée leaves and a couple of thin cornichon slices. Finish with a very light sprinkle of sea salt and an optional pinch of black pepper.

Serve immediately

Arrange tacos on a platter and serve immediately so shells remain crisp and the tartare stays chilled. Provide extra chimichurri at the side.