- In a 3- to 4-quart pot, combine 6½ cups of water, the onions, bay leaf, parsley and 1 teaspoon of salt.
- Bring to a boil over high heat, then add the fish, lower the heat and simmer gently, uncovered, for 6 minutes, until the fish is firm to the touch.
- Remove the fish and cut it into 1-inch chunks.
- Strain the fish stock through a fine sieve set over a bowl, pressing down on the onions and herbs with the back of a spoon before discarding them.
- In a 2- to 3-quart casserole, melt the butter over high heat.
- Add the sliced onions, and cook 6 to 8 minutes until the onions are soft but not brown.
- Stir in the cucumbers and tomatoes and simmer 10 minutes.
- Pour in the strained fish stock, season with the remaining 2 teaspoons of salt and white pepper, and drop in the fish.
- Simmer gently a minute or two, until the soup and fish are heated through.
- Off the heat and stir in the capers, lemon, parsley and.
- Taste for seasoning and serve directly from the casserole.