Bring the broth to a boil in a large saucepan. Pour in the polenta in a steady stream, stirring with a wooden spoon to prevent lumps from forming. Cook over low heat, stirring constantly, for about 5 minutes or until the mixture thickens and pulls away from the sides of the pan. Remove from the heat and stir in the Gruyère cheese. Season lightly with salt and pepper.
Pour the polenta onto a baking sheet coated with cooking spray and spread out into a rectangle measuring about 8 x 7 in. (20 x 18 cm) and about 1⁄2 in. (1 cm) thick. Allow to cool for 1 hour or until set.
Meanwhile, put the dried porcini mushrooms in a bowl and cover with boiling water. Leave to soak for 20 minutes. Drain, reserving 2 tbsp (25 ml) of the soaking liquid. Finely chop the mushrooms.
Preheat the broiler to medium-high. Lightly brush the surface of the polenta with 1 tbsp (15 ml) of olive oil. Cut the polenta rectangle into 12 fingers, each measuring 2 x 21⁄2 in. (5 x 6 cm), trimming the edges to straighten them. Separate the polenta fingers and place them on the same baking sheet. Broil for 4 minutes.
Turn the polenta slices over and broil for an additional 2 to 3 minutes or until lightly browned. Remove from the oven and keep hot.
Heat the remaining 1 tbsp (15 ml) of olive oil in a large non-stick skillet. Add the soaked dried mushrooms and the sliced cremini mushrooms, and sauté over medium-high heat until softened, about 3 to 4 minutes.
Add the sherry and the reserved mushroom soaking liquid. Cook over high heat for 1 to 2 minutes, stirring, until most of the liquid has evaporated. Add the parsley, rosemary, walnuts, and salt and pepper to taste.
Spoon the mushroom mixture on top of the warm polenta fingers. Garnish with a little chopped parsley and serve immediately.
Nutrients per serving (two grills): 246 calories, 108 g fat (2 g saturated fat), 27 g carbohydrates (27 g fibre), 3 g sugar, 7 g protein