Soften butter in a small bowl.

Mince parsley leaves.

Mash anchovy fillet or use anchovy paste.

Combine softened butter, parsley, anchovy paste, and black pepper in the bowl.

Cover the anchovy butter and set aside.

Slice the onion.

In a saucepan, combine sliced onion, white vinegar, sugar, peppercorns, coriander seeds, mustard seeds, salt, and water.

Bring the mixture to a boil.

Simmer for 15 minutes.

Strain the mixture through a fine sieve into a deep skillet.

Add salmon steaks to the skillet.

Bring the liquid to a simmer.

Poach salmon, covered, for 8-10 minutes, or until it just flakes.

Transfer salmon steaks to plates using a slotted spatula.

Drain the poaching liquid off the salmon.

Divide the reserved anchovy butter between the salmon steaks.