2.5-quart french oven, braiser, or dutch ovenTo ensure the fish is properly submerged in liquid, choose a small dutch oven or braiser, or a narrower (9-inch or 10-inch) cast-iron skillet for this recipe.
Zest 1 small orange. Juice the orange into a small bowl and set aside. Remove the root and tough dark green parts of 1 leek. Halve the leek lengthwise, then thinly slice and soak in a bowl of water to remove dirt. Remove from the water and set aside.
Cook the aromatics: Set a small dutch oven or deep skillet over medium heat and add 2 tablespoons extra-virgin olive oil. Add ¼ teaspoon red pepper flakes, ¾ teaspoon fennel seeds, and the orange zest. Once sizzling, stir in 4 garlic cloves, 1 small onion, and 1 leek.Saute for 5 - 7 minutes, or until the aromatics are golden-brown and crisp. Season with a pinch salt and pepper.
Add the tomato paste: Reduce the heat to medium-low, then stir in 3 tablespoons tomato paste. Saute for 2 - 3 minutes, or until the tomato paste is a deep brick red color and the oil has separated.
Reduce the wine and orange juice: Add ¼ cup dry white wine and the orange juice to the pan and stir to combine, scraping up any stuck bits on the bottom of the pan with a wooden spoon. Bring to a boil, then reduce the heat and simmer for 2 - 3 minutes or until about half the liquid has reduced and the alcohol smell has dissipated.
Simmer the broth: Add 1 teaspoon fish sauce (if using) and 1 ½ cups water. Bring back to a boil, then reduce the heat and simmer for 10 - 15 minutes, and stir in 2 tablespoons unsalted butter. At this point the mixture should have thickened slightly and taste aromatic and tomato-ey, but not too acidic or sharp (otherwise, continue simmering to develop the flavors). There may be a thin layer of oil on top of the sauce; this is normal! Season with salt and pepper.Note: Be generous here with the salt, as you want to make sure the broth is well-seasoned for the fish to cook in. Note that the sauce has several layers of acidity from the tomato paste, wine, and orange juice, so a generous pinch of salt will help balance and bring out the savory notes in the dish.
While the sauce simmers, season 2 (6-ounce) cod fillets with salt and pepper on both sides.
Cook the fish: Carefully add the fish to the pan and reduce the heat to a low simmer (there should be small bubbles all around the surface of the pan; if you see large, vigorous bubbles, reduce the heat). Simmer for 3 - 6 minutes until cooked through, meaning a fork inserted into the center of the fish will gently flake the meat without resistance.Note: The fish should mostly be submerged into the liquid, but if it's not, you can occasionally spoon some of the hot liquid over the top as you simmer it.
Serve: Carefully remove fish from the pan with a spatula and divide amongst two serving bowls. Taste and season the broth once more, if needed, then spoon the sauce over the top of the fish. Dollop each bowl with 2 tablespoons Greek yogurt or creme fraiche (if using). Garnish with A handful of flat-leaf parsley and serve immediately with a side of crusty bread.
For a more flavorful broth, try adding seafood stock in place of the water.Not a fan of fennel? Omit it!Recipe inspired by Alison Roman's Tomato Poached Fish