Add fish in single layer and sprinkle with salt. Spoon tomato paste between pieces of fish and cook about 3 minutes, shaking pan occasionally. Stir together vinegar and hot water and add to fish. Bring to boil then lower the heat and simmer, uncovered, 2 to 3 minutes. Carefully turn pieces of fish. Increase heat to medium high and continue to cook just until fish is cooked through, about 10 minutes. Transfer fish and a mounded spoonful of polenta to warm serving plates. Boil sauce vigorously until slightly thickened and correct seasoning with salt, pepper, and a few drops of wine vinegar, if needed. Pour over fish and serve immediately.