Lightly oil a 23 x 30 cm (9 x 12 inch) baking tin with spray oil. Bring the measurement water to the boil in a heavy-based saucepan, add the salt and then gradually whisk in the polenta in a steady stream. Cook over a low heat, stirring constantly with a wooden spoon, for 5 minutes until the grains have swelled and thickened.
Remove from the heat and immediately beat in the garlic, butter, Parmesan and pepper until smooth. Pour the mixture into the prepared tin and leave to cool.
Turn the polenta out on to a chopping board and cut into large squares, then diagonally in half into triangles. Brush the triangles with a little oil.
Heat a ridged griddle pan until hot. Add the polenta triangles, in batches, and cook over a medium-high heat for 2–3 minutes on each side until charred and heated through. Serve immediately, garnished with grated Parmesan and chopped parsley.