Preheat your oven to 400°F (200°C).

In a spice grinder or mortar and pestle, combine the allspice berries and fresh rosemary leaves. Crush them until they form a coarse mixture.

In a small bowl, mix the crushed allspice and rosemary with the minced garlic, olive oil, salt, and black pepper to create a paste.

Rub the spice paste all over the beef tenderloin, ensuring an even coating. Allow the tenderloin to sit at room temperature for about 30 minutes to marinate.

Heat a large oven-safe skillet over medium-high heat. Once hot, add the tenderloin and sear for about 4-5 minutes on each side until browned crust forms.

Transfer the skillet to the preheated oven and roast the tenderloin for 20-25 minutes for medium-rare, or until it reaches your desired doneness. An internal thermometer should read 130°F (54°C) for medium-rare.

While the beef is roasting, prepare the caramelized shallot butter. In a separate skillet, melt the unsalted butter over medium heat.

Add the sliced shallots and sauté for about 10-12 minutes, stirring occasionally, until the shallots are golden brown and caramelized.

Stir in the balsamic vinegar and cook for an additional 2 minutes. Remove from heat and set aside.

Once the tenderloin is cooked, remove it from the oven and let it rest for 10 minutes before slicing.

Serve the sliced tenderloin drizzled with the caramelized shallot butter and garnish with fresh rosemary sprigs.