Bring the stock to the boil in a saucepan, then reduce to a simmer. Stirring continuously, pour in the polenta in a steady steam and cook for 5 mins until thickened. Stir in the basil and season with black pepper and salt, if you like. Spread on an oiled shallow tin measuring 24 x 18cm. Leave to set for 1 hr.
Cut the polenta into 9 rectangles, each 8 x 6cm, then cut in half diagonally to make triangle shapes. Heat a griddle until hot, brush each triangle with oil and grill for 4-5 mins each side, until crisp and golden.
Top each triangle with 1⁄2 tsp tapenade and half a tomato. Serve warm on salad leaves.