You can butter poach virtually any other fish. Trout is an obvious alternative, but you could use just about any fish you can cut into blocks: Cod, halibut, grouper, cobia, shark, swordfish or sturgeon, a large striped bass, etc. Play around with it and see what you like best.
4portions of skinless salmon,for a total of 1 to 1 1/2 pounds
1/4cupkosher salt(I use Diamond Crystal)
1/4cupunsalted butter OR 2 whole sticks(see below)
2to 3 tablespoons minced dill
2to 3 teaspoons prepared horseradish
Black pepper to taste
Dissolve the salt in the water and brine the salmon in the refrigerator for 30 minutes to 1 hour, depending on how thick it is. Move the salmon to a cutting board and pat dry with paper towels.
While the salmon is brining, mix together the sour cream, horseradish and dill in a bowl.
If you have a vacuum sealer, seal each portion of salmon in its own bag -- if you jam them all together they won't cook as well -- each with at least 1 tablespoon of butter; you can use more if you'd like. If you don't have a vacuum sealer, melt the butter in a pot large enough to submerge at least 1 piece of salmon. Turn the oven on to "warm" and put a baking sheet inside; this is for the cooked pieces of salmon to rest while you do the others. Of course, you could also melt a pound or so of butter and poach all of the salmon at once, but that gets expensive.
The easiest way to poach the salmon is to cook it sous vide for 20 to 45 minutes at 135°F -- the lower end of this spectrum will give you rare-on-the-inside salmon, the upper end will get you fully cooked but still luscious salmon. If you don't have a sous vide water oven., fill a large pot 3/4 full of water and heat it to the steaming point, about 150°F to 160°F. Don't let it simmer. Drop the vacuum-sealed salmon in the water, cover the pot and turn off the heat. Let them cook in the bags for 10 to 20 minutes. If you are doing the butter bath method, melt the butter over low heat. Place the salmon in the butter and let it cook gently for 10-20 minutes. Carefully lift it out with a spatula and move it to the preheated oven.
Once your salmon is poached, serve with a dollop of the sour cream and top with black pepper. Roasted or boiled potatoes are a good accompaniment.