Heat a braiser or straight-sided skillet over medium-high heat. Add the olive oil, garlic, red onion, fennel and pepper flakes to the pan. Season with 1/2 teaspoon salt. Cook, stirring often with a wooden spoon, for about 5 minutes, or until the vegetables are soft and fragrant. Deglaze with the wine. Stir, scraping up any bits from the bottom of the pan, and simmer until reduced by half, about 2 minutes. Add the tomatoes, water and capers. Bring to a simmer, then reduce the heat to medium low to maintain a gentle simmer and cook 10 minutes.
Season the fish with the remaining 1/2 teaspoon salt. Slide the fish into the broth and cover with a tight-fitting lid. Cook for 12 to 15 minutes, or until the fish is firm and opaque all the way through. Remove the lid and spoon some of the sauce over the fish. Sprinkle with parsley, drizzle with olive oil and serve with plenty of crusty bread to soak up the broth.
You can use any firm white fish for this, such as sole or halibut.