1.Grease 20cm x 30cm (8 inch x 12 inch) lamington pan.
2.Bring milk to the boil in medium saucepan, gradually stir in polenta. Cook, stirring, about 10 minutes or until polenta thickens. Stir in cheeses and spinach. Spread polenta mixture into pan, cover, refrigerate 2 hours or overnight or until firm.
3.Preheat oven to 200°C (180°C fan forced).
4.Turn polenta onto board, cut into 30 squares. Place polenta onto baking-paper (parchment) lined oven tray. Bake about 20 minutes or until browned lightly.
5.Meanwhile, combine tomato, vinegar and oil in small baking dish. Roast, alongside polenta, about 15 minutes or until tomato softens slightly.
6.Serve polenta squares topped with tomato halves, drizzle with tomato pan juice.