In a small cup, pour boiling water over sun-dried tomatoes to reconstitute.

Heat 1 tablespoon oil in a skillet over medium heat. Add mushrooms, garlic, salt and rosemary. Cook, stirring, for about 5 minutes.

Place mushroom mixture, sun-dried tomatoes, walnuts, parsley and remaining 2 tablespoons oil in a food processor. Pulse until the mixture is the desired consistency. Slice baguette into about 24 thin slices. Brush each slice with olive oil.

Add 1 to 2 teaspoons of mushroom mixture to each slice. Top with Parmesan, if using. Put crostini on a baking sheet and place under a broiler set on low. Broil until the edges of bread begin to brown. Watch carefully so they don’t burn!