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1
Place the porcini mushrooms in a bowl of hot water; press to submerge.
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2
Let stand until the mushrooms are tender, about 15 minutes.
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3
Scoop out mushrooms, being carefully not to stir any dirt that may have sunk to the bottom of the water.
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4
Discard mushroom water.
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5
Combine the porcini mushrooms, button mushrooms, walnuts, garlic, and parsley in a food processor and pulse until coarsely chopped.
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6
With the machine running, gradually add 1/2 cup of the oil, blending just until the mushrooms are finely chopped.
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7
Transfer the mushroom mixture to a medium bowl.
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8
Stir in the Parmesan.
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9
Season the pesto with salt and pepper, to taste.
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10
If not using mushroom pesto right away, cover tightly with plastic wrap to prevent possible discoloration of mushrooms.
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11
Preheat a grill pan to medium-high heat.
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12
Arrange the bread slices on pan, cut-side down.
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13
Brush the remaining 1/4 cup of oil over the bread slices.
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14
Cook until pale golden and crisp, about 5 minutes.
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15
Alternatively, you can toast the bread in the oven.
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16
Preheat the oven to 375 degrees F.
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17
Arrange the bread slices on 2 heavy large baking sheet.
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18
Brush the remaining 1/4 cup of oil over the bread slices.
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19
Bake until pale golden and crisp, about 15 minutes.
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20
Spread the mushroom pesto over the crostini.
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21
Arrange the crostini on a platter and serve.
