Preheat the oven to 325°F (160°C).

In a dry skillet over medium heat, toast the coriander seeds until fragrant, about 2-3 minutes. Remove from heat and let cool, then crush lightly using a mortar and pestle or a spice grinder.

In a small bowl, combine the toasted coriander, cracked green peppercorns, minced garlic, chopped rosemary, thyme, olive oil, salt, and black pepper. Mix well to form a paste.

Rub the coriander and peppercorn paste all over the lamb shoulder, ensuring it is evenly coated. Place the lamb shoulder in a roasting pan and cover it loosely with aluminum foil.

Roast the lamb in the preheated oven for about 3 to 4 hours, or until the meat is tender and easily pulls apart with a fork. Remove the foil for the last 30 minutes to allow the crust to develop.

While the lamb is roasting, heat a skillet over medium heat and add the sliced onions. Cook slowly, stirring occasionally, until the onions are caramelized and golden brown, about 20-25 minutes.

In a mixing bowl, combine the caramelized onions with softened butter, balsamic vinegar, honey, and a pinch of salt. Mix until well combined.

Once the lamb is done, remove it from the oven and let it rest for 15 minutes before slicing. Serve the lamb slices topped with the caramelized onion confit butter.