For me, there’s something likable, but not quite lovable, about poached fish: the ultraclean flavor, the melt-away texture, the no-fat virtuousness. But the result is oftentimes bland–a little too clean and virtuous, like spa food. After a solid day of fishing, I’m craving something hearty. That’s where a jug of buttermilk comes in. Poaching fish in buttermilk yields the luscious texture and pure flavor, but with a more substantial richness and a poaching liquid you’ll want to lap up with a spoon.
SEASON THE FILLETS with salt and pepper. Combine the fish with the buttermilk and tarragon, in a single layer, in a large saute pan over medium heat. When the buttermilk begins to simmer, cover the pan and cook 2 minutes. Gently roll the fillets over, and cook another minute until the fish is firm but not falling apart.
WITH A SLOTTED SPATULA, remove the fillets to a plate and keep warm, either by covering with foil or by placing in a warm oven. Remove the tarragon sprig and discard. Transfer the buttermilk, which will have separated during cooking, to a blender and add the remaining 4 Tbsp. of butter. (If you have one of those handheld immersion blenders, this is a prime time to use it.) Blend until the butter is smoothly incorporated and the buttermilk is no longer separated. Add the lemon juice, and salt and pepper to taste
IN A LARGE SAUTÉ PAN, heat 1 Tbsp. of butter over medium-high heat. Cook the bacon until most of the fat is rendered and the bacon is almost crisp. Add the greens, and toss them using tongs until they begin to wilt. Add about 1/2 cup of water and cook, covered, stirring occasionally, for 5 minutes. Remove the cover and continue to cook until the mustard greens are just tender and most of the liquid has evaporated. Season with salt and pepper, and keep warm until ready to serve.
PLACE THE GREENS in a shallow bowl and top with the fish. Pour some of the buttermilk over and around the fish. Sprinkle with the chives and serve.
1 Tbsp. plus 4 Tbsp. butter
2 slices bacon, preferably thick-cut, diced
1 1/2 lb. (2 bunches) mustard greens, stems and center ribs discarded and leaves halved
1/2 cup water
4 6-oz. fillets of trout, walleye, pike, or other fish
3 cups buttermilk
1 sprig fresh tarragon
Juice of one lemon, to taste
1 Tbsp. fresh chives, finely chopped
Salt and freshly ground black pepper, to taste