Place the milk and sage leaves in a medium saucepan over high heat and bring to a boil. Remove sage leaves. Gradually whisk in the polenta and reduce the heat to low. Cook, stirring, for 4 minutes, or according to package instructions, until thickened.
Remove from the heat, add the Parmigiano Reggiano, salt and pepper and mix to combine. Spread the polenta onto an oven tray covered with waxed baking paper and spread out to a thick circle. Top with mozzarella cheese. Place under the grill and cook for 5-6 minutes or until the cheese is well melted.
In the meantime, heat the Portobello Mushroom sauce according to package instructions. Spread the sauce over the polenta, garnish with sage leaves and serve.