Preparation

• Heat a small, heavy skillet over high heat for 1 minute. Season the beef with 1/4 tsp. salt and some pepper. Turn the exhaust fan to high and sear the beef, flipping after 3 minutes, and then cook, flipping every couple of minutes until it’s done to your liking, about 10 minutes total for medium rare. Transfer to a cutting board, let rest for 5 minutes, and then slice the beef thinly.
• Mix the crème fraîche and horseradish in a small bowl; season with 1/4 tsp. salt and a few grinds of pepper.
• To assemble, top the crostini with a slice of beef, a dollop of the crème fraîche, and a small piece of frisée and serve.

Make Ahead Tips

The beef can be cooked a few hours ahead and refrigerated. Bring it to room temperature and slice just before serving.