In a large saucepan of boiling water boil the lentils for 15 to 20 minutes, or until they are just tender, drain them in a sieve, and rinse them briefly under cold water. Drain the lentils well and in a bowl toss them with the bell pepper, the onion, the tomatoes, the coriander, the oil, the vinegar, and salt and black pepper to taste.
Pat the fillets dry, season them with salt and pepper, and dredge them in the flour, shaking off the excess. Grill the fillets on an oiled rack set about 6 inches over glowing coals or in an oiled ridged grill pan over moderately high heat for 4 to 5 minutes on each side, or until the just flake. Divide the salsa among 4 plates and top each serving with a fillet.