In a 5- to 6-quart pan, combine polenta and broth. Stir over high heat until mixture boils, then reduce heat and simmer, stirring often, until polenta is smooth to taste, about 15 minutes.
Meanwhile, rinse fish and pat dry; cut into 4 equal pieces. Sprinkle lightly all over with salt and pepper. Lay salmon skin side up in a lightly oiled 10- by 15-inch pan.
Broil about 4 inches from heat for 4 minutes. With a wide spatula, turn salmon over and broil until fish is opaque but still moist-looking in center of thickest part (cut to test), 3 to 4 minutes longer.
Meanwhile, in a small bowl, mix corn, pimientos, onion, lime juice, and mint.
Spoon polenta equally onto plates. Add a piece of salmon to each and garnish with corn relish. Add more salt and pepper to taste.