1. In 12-inch skillet over medium heat, heat salt, 3/4 cup water, and 3 tablespoons butter or margarine to boiling. Add endive; heat to boiling; cover and cook 5 minutes. With slotted pancake turner, remove endive to plate.
2. Meanwhile, in 1-quart saucepan over medium heat, into 2 tablespoons hot butter or margarine (1/4 stick), tear bread into crumbs. Cook, stirring frequently, until bread crumbs are browned. Remove from heat.
3. Arrange fish fillets in simmering liquid in skillet; place an endive half on each fillet; sprinkle endive with lemon juice. Cover skillet and simmer 3 to 4 minutes, just until fish flakes easily when tested with a fork and endive are fork-tender. With slotted pancake turner, remove endive-topped fillets to warm platter, draining as much liquid back into skillet as possible; reserve liquid.
4. In cup, mix flour and 1/4 cup water until smooth; gradually stir into hot liquid in skillet and cook over medium heat, stirring constantly, until mixture boils and thickens slightly. Stir in capers; press with spoon to blend flavors. Pour sauce over fish and endive; sprinkle crumbs on top of endive.