For maximum flavor, we cook the salmon in the time-honored way, by poaching it gently in well-salted boiling water. Better still, use sea water if you are close to the coast.
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The proportion of salt to water is very important. We use 1 rounded tablespoon salt to every 5 cups water. Although the fish or piece of fish should be just covered with water, the aim is to use the minimum amount of water to preserve the maximum flavor, so therefore one should use a saucepan that will fit the fish exactly. An oval cast-iron saucepan is usually perfect.
Half fill the pan with measured salted water and bring to a boil. Put in the piece of fish, just covering with water, and bring back to a boil. Simmer gently for 20 minutes.
Turn off the heat, let the fish sit in the water. Serve within 15–20 minutes. Meanwhile, make the Irish butter sauce.
Put the egg yolks into a heavy-bottomed stainless-steel saucepan on a very low heat. Add the cold water and whisk thoroughly.
Add the butter bit by bit, whisking all the time. As soon as one piece melts, add the next. The mixture will gradually thicken, but if it shows signs of becoming too thick or "scrambling" slightly, remove from the heat immediately and add a little cold water if necessary. Do not leave the pan or stop whisking until the sauce is made. Finally, add the lemon juice to taste. Pour into a bowl and keep warm over hot, but not boiling, water.
To serve, lift the cooked salmon carefully from the poaching liquid. Peel off the skin gently. Garnish with sprigs of watercress or parsley. Serve with the Irish butter sauce.