Turn oven on to a low broil. Layer crostini slices on a large cookie sheet. Drizzle with olive oil, and season with seasalt and cracked peppercorn.
Toast in the oven for about 3 minutes (keep a close eye on it) - you only want golden brown so your crostini’s aren't too crispy.
Wagyu Beef (Based on 3/4 Inch Steak)
Allow the steak to rest on the counter for about half an hour. Preheat a frying pan over high heat.
Season the steak generously with salt and pepper. Pour olive oil into the pan.
Add steak to the pan and sear for 3-4 minutes. Add butter and smashed garlic cloves to the pan and flip the steak. Sear for another 3-4 minutes. Spoon butter over the steak. Remove from the heat and allow to rest for 10-15 minutes on a cutting board. After resting, slice thin.
Mix all ingredients together. Stir.
Pour olive oil and butter into a pan over medium heat. Add shallots. Cook down until brown and fragrant.
Add horseradish mayo to the toasted crostini. Then layer caramelized shallots, arugula, sliced wagyu, and horseradish. Garnish with optional chive.