Read through the steps first. Gather all ingredients as the béarnaise benefits from warm ingredients and a calm cook.

Combine white wine vinegar, white wine (or water), shallot, and half of the chopped tarragon in a small saucepan.

Bring to a simmer over medium heat and reduce until about 2 tablespoons of liquid remains.

Strain the solids into a small bowl and let the reduction cool to warm.

Pat the beef tenderloin dry and season generously with kosher salt and pepper.

Heat a heavy skillet over medium-high heat and add olive oil.

Sear the tenderloin on all sides until browned, about 2–3 minutes per side.

Transfer to a baking sheet and roast in a preheated oven at 400°F for 8–12 minutes until the internal temperature reaches about 125°F for medium-rare.

Let the beef rest for 10 minutes, then slice thinly against the grain.

Preheat oven to 375°F.

Brush baguette slices lightly with melted butter on both sides and place on a baking sheet.

Toast for 8–10 minutes, flipping once, until golden and crisp.

While still warm, rub each slice with the halved garlic.

In a heatproof bowl, whisk the egg yolks with the warm tarragon reduction.

Set over barely simmering water and whisk until the yolks thicken slightly.

Slowly drizzle in warm melted butter while whisking to form a smooth emulsion.

Stir in lemon juice, remaining tarragon, and season with salt and white pepper. Keep warm until ready to assemble.

Place a slice of beef on each crostini, spoon a dollop of béarnaise over the beef, and finish with chopped chives or microgreens and a sprinkle of flaky sea salt.

Serve immediately to keep the bread crisp.