Let polenta cool thoroughly. If eating dinner in a matter of hours (or sooner), place in the freezer for 15 to 20 minutes to facilitate firmness. If you have more time, cover and place in the refrigerator for 1 to 2 hours, or until firm.
Before serving, brush polenta lightly with olive oil. Place under the broiler for 1 to 2 minutes, until slightly browned and crispy.
Cut into 4 to 6 triangles. Ladle tomato sauce over polenta.