Preheat your oven to 400°F (200°C).

In a dry skillet over medium heat, toast the allspice berries until fragrant, about 3-4 minutes. Remove from heat and allow to cool slightly. Once cooled, crush the allspice berries using a mortar and pestle or a spice grinder.

In a small bowl, combine the crushed allspice, chopped wild rosemary, olive oil, salt, and black pepper to create a paste.

Pat the beef tenderloin dry with paper towels. Rub the allspice and rosemary paste all over the tenderloin, ensuring an even coating.

Heat a large oven-safe skillet over medium-high heat. Sear the tenderloin on all sides until browned, about 2-3 minutes per side.

Transfer the skillet to the preheated oven. Roast the tenderloin for about 25-30 minutes, or until it reaches your desired level of doneness (145°F for medium-rare). Remove from the oven and let it rest for 10 minutes before slicing.

While the tenderloin rests, prepare the caramelized leek confit butter. In a separate saucepan, melt the unsalted butter over medium heat. Add the sliced leeks and cook slowly, stirring occasionally, until the leeks are soft and caramelized, about 15-20 minutes.

Stir in the honey and balsamic vinegar, and cook for an additional 2-3 minutes. Remove from heat.

To serve, slice the tenderloin into medallions and arrange them on a platter. Spoon the caramelized leek confit butter over the top and garnish with fresh rosemary sprigs if desired.