White Fish Braised with Tomato, Fennel, and Potatoes

Servings 4

3 tablespoons olive oil
1 small yellow onion, halved and cut into 1/4-inch half-moons
1 small fennel bulb, halved, cored, and cut into 1/4-inch half-moons, reserving a few fronds
Kosher salt and freshly ground black pepper
Pinch of red pepper flakes
1 cup gluten-free chicken or vegetable broth
1 cup tomato sauce or tomato puree
8 small red or multicolored potatoes, sliced
Zest and juice of 1 lemon or 1 small orange
3 sprigs fresh thyme
4 (6-ounce each) lingcod fillets or other firm-fleshed white fish fillets

In a 12-inch heavy skillet over medium-high heat, warm the oil. Add the onion, fennel, 1/2 teaspoon salt, a grinding of black pepper, and the red pepper flakes. Cover and cook, stirring occasionally, until the vegetables are tender, 7 to 8 minutes. Add the broth, tomato sauce, potatoes, lemon zest and juice, thyme, and 1/2 teaspoon salt. Cover and bring to a boil. Lower the heat and simmer, covered, for 10 minutes. Season with pepper. Tuck the fish fillets into the sauce in one layer. Cover and simmer for 10 minutes, gently turning the fish halfway through cooking. Remove and discard the thyme sprigs, garnish with the reserved fennel fronds, and serve.