Pat the flank steak dry with paper towels. In a small bowl, combine kosher salt, black pepper, garlic powder, and onion powder. Rub the steak all over with 1 tablespoon of olive oil, then sprinkle generously with the spice mixture. Let it rest at room temperature for at least 15 minutes, or up to 30 minutes.

Heat a large cast-iron skillet over high heat until smoking lightly. Add the seasoned steak to the hot skillet. Sear for 3-4 minutes per side for medium-rare, or longer for desired doneness.

Remove the steak from the skillet and transfer it to a cutting board. Tent loosely with foil and let it rest for 5-10 minutes. Resting is crucial for juicy steak. Once rested, slice the steak thinly against the grain. Aim for bite-sized pieces or strips that can be folded onto the crostini.

Preheat oven to 375°F (190°C). Arrange the baguette slices in a single layer on a baking sheet. Brush or drizzle the baguette slices lightly with 2 tablespoons of extra virgin olive oil and sprinkle with 1/4 teaspoon of kosher salt.

Bake for 8-12 minutes, or until golden brown and crispy. Keep a close eye on them to prevent burning. Let cool slightly. (Alternatively, for an air fryer, set to 350°F and cook for 4-6 minutes, checking frequently).

Spread a generous amount of room-temperature Boursin Garlic & Fine Herbs cheese onto each toasted baguette slice.

Top each crostini with a thin slice or folded piece of the rested, thinly sliced steak. Garnish with finely chopped fresh chives just before serving.

Serve immediately.