Using a vegetable peeler, remove zest from 2 of the lemons and juice the zested lemons. Transfer juice and zest to a straight sided skillet or sauce pan with a tight fitting lid. Add stock, garlic, and 4 sprigs of thyme. Bring to a boil.
Salt both sides of the halibut and carefully place into the skillet. Cover the pan and turn off the heat. Let the fish stand in the stock for about 6 to 8 minutes until the fish is opaque white and firm to the touch. Transfer the fish to a heated platter and tent with foil to keep warm.
Pour the broth through a cheesecloth lined sieve, discard solids. Return the broth to the skillet and bring to a boil, then turn heat off.
Dived the broth between wide bowls and gently place a fillet in each. Cut remaining lemon into wedges. Garnish with a lemon wedge, sprigs of thyme and a drizzle of olive oil.