Preheat oven to 400°F (200°C). Brush both sides of each baguette slice lightly with olive oil. Arrange on a baking sheet and toast for 6–8 minutes, flipping halfway, until golden and crisp.

If you like garlicky crostini, rub the warm toasts with a halved garlic clove once they come out of the oven.

In a small saucepan over medium heat, combine the shallot, white wine vinegar, white wine (or water), and half the tarragon. Reduce until only about 1 tablespoon of liquid remains.

In a heatproof bowl, whisk the egg yolks with the warm reduction. Place the bowl over a pot of gently simmering water (double boiler) making sure the bowl doesn’t touch the water. Whisk constantly until the yolks thicken and become pale.

Slowly drizzle in the warm melted butter while whisking constantly, until the sauce is thick and glossy. Stir in lemon juice, remaining tarragon, and season with salt and white pepper.

If the sauce becomes too thick, whisk in a teaspoon of warm water. Keep warm but off the heat.

Pat the tenderloin dry and season generously with salt and pepper. Heat a heavy skillet (cast iron is ideal) over medium-high heat with the butter.

When foaming, sear the tenderloin on all sides until nicely browned, about 2–3 minutes per side. For medium-rare, cook until an instant-read thermometer reads 125–130°F.

Remove the beef to a cutting board and let it rest for 8–10 minutes. Slice thinly against the grain into medallions.

Place a slice of beef on each toasted baguette round. Spoon a small dollop of béarnaise over the beef.

Garnish with chopped chives or a tiny sprig of tarragon and a pinch of flaky sea salt.

Arrange on a platter and serve immediately.