Place the porcini mushrooms in a bowl of hot water; press to submerge. Let stand until the mushrooms are tender, about 15 minutes. Scoop out mushrooms, being carefully not to stir any dirt that may have sunk to the bottom of the water. Discard mushroom water.
Combine the porcini mushrooms, button mushrooms, walnuts, garlic, and parsley in a food processor and pulse until coarsely chopped. With the machine running, gradually add 60ml of the oil, blending just until the mushrooms are finely chopped.
Transfer the mushroom mixture to a medium bowl. Stir in the Parmesan. Season the pesto with salt and pepper, to taste. If not using mushroom pesto right away, cover tightly with plastic wrap to prevent possible discoloration of mushrooms.
Preheat a griddle pan to medium-high heat. Arrange the bread slices on pan, cut-side down. Brush the remaining oil over the bread slices. Cook until pale golden and crisp, about 5 minutes.
Alternatively, you can toast the bread in the oven. Preheat the oven to 190°C/gas mark 5. Arrange the bread slices on two heavy, large baking sheets. Brush the remaining 60ml of oil over the bread slices. Bake until pale golden and crisp, about 15 minutes.
Spread the mushroom pesto over the crostini. Arrange the crostini on a platter and serve.