Heat the oven to 350 F. Spray 4 squares of foil with cooking spray.
Place 1 cod fillet on each foil square. Cut lemon in half. Squeeze the juice from one half over the fish. Cut the other half into slices, place over the fish and seal the foil.
Bake in the oven until the fish is opaque throughout when tested with the tip of a knife, about 20 minutes.
In a small bowl, add the chicken bouillon granules and the hot tap water. Stir until the granules dissolve. Set aside.
In another small bowl, mix the butter and flour together. Transfer to a heavy saucepan. Stir over moderate heat until the butter-flour mixture melts. Add the bouillon to the butter mixture and continue to stir until thickened. Add the capers and remove from the heat. Pour over the fish and serve.