Bring the stock to the boil in a medium-sized saucepan. Add the polenta in a steady stream, stirring constantly. Cook for about 5 minutes or according to the packet instructions, until the mixture has thickened and is pulling away from the sides of the saucepan.
Stir in the parsley and oregano or marjoram then season with pepper. Pour the polenta onto a greased baking tray and spread out to make a rectangle about 20 x 18 cm and 1 cm thick. Leave in a cool place to set for about 1 hour.
Preheat the grill to high. In a bowl, mash the Gorgonzola with the ricotta cheese and set aside. Brush the top of the polenta lightly with the oil. Cut into 12 rectangles, each about 5 x 6 cm. Place on the rack of the grill pan, oiled-side up then grill for 8–10 minutes until the tops are lightly browned.
Turn the polenta slices over and top with the Gorgonzola mixture. Grill for a further 2–3 minutes or until the cheese has melted and is beginning to brown. Serve at once, garnished with mixed salad leaves.