1 tablespoon unsalted butter
½ small onion, ¼ inch slices
1 Yukon Gold potato, very thinly sliced
2 (6-ounce) fish fillets (halibut, cod or snapper)
1 cup white wine (more or less)
1 lemon, thinly sliced
1 tablespoon herbes de Provence
1 sprig fresh thyme
2 bay leaves
3-4 sprigs parsley, finely mince leaves and reserve stems
3-4 Campari tomatoes, sliced
1 cartouche ( see note)
Spread the butter in the bottom of a pan of the smallest diameter to fit your fish not touching each other. Place the onion and potato slices on the bottom. Place the fish fillets on top. Sprinkle lightly with sea salt, and a little fine ground black pepper if desired. Add the wine, adjusting the amount as necessary so the depth comes about a quarter of the way up the fish. Reserve a few lemon slices for garnish and place the remaining lemon slices on the fish, and add parsley stems, bay, thyme and tomatoes around the sides of the fish.
Cover the fish with a buttered cartouche, gently tucked into the sides of the pan. Bring the liquid to a gentle simmer and cook, checking occasionally to be sure the fish is not boiling and adjusting the heat to maintain the gentlest simmer. When the fish reaches an internal temperature of 125° F to 140° F, about 10-15 minutes depending on the thickness, use a flat spatula to transfer the fish to a plate with the onions, tomatoes and potatoes. Cover with the cartouche to keep warm. Remove the herbs from the broth and discard them. Increase the heat to medium-high to reduce the liquid until it becomes syrupy or just thinly covers the bottom of the pan. You need only a couple of tablespoons. Remove from heat and whisk in up to 2 Tablespoons of unsalted butter, a small piece at a time, until you have a velvety sauce. Whisk in 1 tablespoon finely minced parsley.
Serve the fish on the bed of veg, top with the wine sauce and garnish with the reserved lemon slices. If you like, chop another tomato and garnish the dish with pieces of bright red tomato.
Note: If you have heavy cream on hand, you may whisk in 3-4 tablespoons of cream to the reduction, off the heat, in place of the butter.
To make cartouche for poaching:
Start with a square of parchment paper. Fold in half and then crosswise in half again to form a square ( -ish, is okay). Then with the point facing to the left, fold upwards two more times to form a long skinny triangle. Eyeball the point of the parchment at the center of your pan and you’re going to cut the other end of the paper at the point where the inside edge of the pan is. Then cut off the tip about ½ from the point to make a hole, unfold it and place it on top of your fish.
What does a cartouche do?
A cartouche controls evaporation, allowing your cooking liquid to remain at full strength while steam can escape. It maintains your temperature where a lid would increase temperature. When you want to cook low and slow, a parchment cartouche is the perfect technique.