Line the base and sides of a 4cm-deep, 16cm x 26cm (base) slab pan with baking paper.
Bring the stock and water to the boil in a saucepan over medium heat. Pour in the polenta, in a steady stream, whisking constantly. Cook, stirring with a wooden spoon, for 5 minutes or until thick.
Stir in the parmesan.
Spread the hot polenta into the prepared pan. Smooth the surface.
Cover and refrigerate for 3 hours or until firm.
Combine the vinegar and sugar in a small saucepan over high heat. Bring to the boil. Reduce heat to medium and simmer for 5 minutes or until thick and syrupy.
Heat a non-stick frying pan over medium-high heat. Cut polenta into triangles. Lightly spray with oil. Cook, in batches, for 34 minutes on each side or until golden.
Lightly spray a large frying pan with olive oil. Heat over medium-high heat. Add the garlic and mushrooms. Cook for 3 minutes or until tender. Add the olives and tomatoes. Cook for 23 minutes or until warmed through. Remove from heat.
Stir in the parsley. Place polenta on plates. Top with mushroom mixture. Drizzle with balsamic glaze. Serve.