Broil the Steak:

1. Preheat the broiler for at least 5 minutes, and line a broiler pan with aluminum foil.
2. In the meantime, pat the flank steak dry using paper towel. Sprinkle both sides with Montreal steak spice, gently pressing in to make a crust.
3. Place the steak on the prepared broiler pan and broil for 4-5 minutes per side, until cooked to medium-rare or a meat thermometer registers an internal temperature of 165F. Tent with aluminum foil and set aside to rest for 5 minutes, or until ready to serve.

Prepare the Aioli:

1. While the steak is broiling, prepare the aioli.
2. Using a large heavy knife, finely mince the garlic. Sprinkle with salt, then use the flat of the knife to mash the salt and garlic together into a paste. (You can also do this step using a mortar and pestle, if you have one.)
3. Scrape the garlic paste into a small bowl. Add egg yolk, and whisk until smooth. Begin pouring in the olive oil in a thin, steady stream as you whisk briskly, continuing until the oil is completely incorporated and the aoili is thick and smooth. (If the aioli starts to separate at any point during this process, stop adding oil immediately. Whisk the aoili briskly until it comes together again, then continue on as usual.)
4. Stir in horseradish. Taste and adjust seasoning with more salt if necessary. Cover tightly with plastic wrap and refrigerate until ready to use.

Assemble the Crostini:

1. To assemble, arrange the baguette slices on a serving platter. Spread each one with a small dollop of horseradish aioli, then top with a few baby arugula leaves.
2. Cut flank steak into thin slices across the grain. Lay a slice on top of each crostini. Serve immediately.