Set up the poaching liquid: Add 0.5 cups water or light broth for the pan to a wide skillet with a lid. Lay 4 fresh thyme sprigs (or a small pinch of dried) in the liquid. Bring to a bare simmer over medium-low heat.
Add fish: Season 2 cod fillets (6 oz each), or sole if available with 0.3 teaspoons salt on both sides. Lay fillets in the simmering liquid. Place 1 lemon, thinly sliced slices on top.
Poach covered: Cover and cook until fish is opaque throughout and flakes easily with a fork. Thinner fillets like sole will be closer to 6 minutes; thicker cod closer to 10.
Serve: Lift fillets out with a wide spatula. Serve immediately with lemon from the pan squeezed over top.