Simmer poaching liquid: In a large skillet, combine carrots, celery, onion, lemon, 1 1/2 teaspoons salt, and 6 cups water. Bring to a boil; reduce to a simmer, cover, and cook 8 minutes.
STEP 2
Add salmon to poaching liquid: Season salmon with salt and gently lower into the simmering liquid (liquid should just cover fish). Reduce to a very gentle simmer.
STEP 3
Cook salmon: Cover and cook until salmon is opaque throughout, about 5 minutes (longer for thicker fillets).
STEP 4
Remove and serve: Using a wide slotted spatula, remove salmon from liquid.