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.