Bring lamb to room temperature. Preheat oven to 275° F.
Place salt and pepper in small bowl.
Toast coriander seeds in dry skillet over medium heat until aromatic.
Crack ⅔ of seeds. You can do this in a mortar, or you can place the seeds between two pieces of parchment paper and run a rolling pin over them. Add cracked seeds to salt and pepper.
In spice grinder or mortar, grind remaining seeds until fine. Add to cracked seeds, salt, and pepper.
Insert garlic slices into roast. Push point of paring knife about 1 inch into fleshly part of lamb. Slide a slice of garlic into lamb alongside the knife blade. Repeat until all garlic slices are used.
Rub coriander mixture all over the roast. Place roast in roasting pan and drizzle all over with olive oil.
Roast about 6 hours, until tender and brown all the way through. The internal temperature should be about 165° F.
Let rest, covered, for 15 minutes before carving.