Special equipment: Pastry Brush Squeeze Bottle
Preheat the oven to 350 degrees F.
Lay the baguette slices on a baking sheet. Combine the butter and olive oil in a small bowl, and liberally brush each slice with the butter mixture. Bake in the top half of the oven until slightly golden, about 15 minutes.
Rub the tenderloin with the garlic, pepper, and salt. Spray a large oven-proof skillet with cooking spray and place it over high heat for 1 minute. Put the tenderloin into the skillet and brown it on all sides, about 1 minute per side. Transfer the skillet to the oven and roast the tenderloin until an instant-read thermometer inserted in the center registers 120 degrees F for rare, about 15 to 20 minutes. Remove the beef to a cutting board and rest for at least 15 minutes, or until ready to use. Immediately before serving, slice the tenderloin thinly into about 30 pieces.
To assemble the hors d'oeuvre, squeeze a small dollop of the whipped cream onto the crostini. Top with a rosette of tenderloin and another dollop of whipped cream. Garnish with a sage leaf and serve.
Whisk the heavy cream in a well-chilled medium mixing bowl until soft peak. Add the sage, salt and pepper and whisk until the peaks are firm. Spoon the whipped cream into the squeeze bottle and refrigerate until ready to use.