Using a vegetable peeler, remove zest in strips from 2 lemons. Juice zested lemons. Transfer zest and juice to a 12-inch straight-sided skillet with a tight-fitting lid. Add stock, garlic, 4 sprigs thyme, and the peppercorns to the pan. Bring to a boil.
Season each fillet on both sides with salt; carefully add to skillet. Cover skillet, and turn off the heat. Let stand until fish is opaque and firm to the touch, 9 to 12 minutes. Using a slotted spatula, transfer fish to a platter; tent loosely with foil to keep warm.
Pour poaching liquid through a cheesecloth-lined sieve into a bowl; discard solids. Return to skillet. Bring to a boil; turn off heat.
Divide broth among wide bowls; gently place a fish fillet in each. Cut remaining lemon into wedges. Garnish each serving with a wedge of lemon and a sprig of thyme, and lightly drizzle with oil.