Thinly slice the baguette. Place the rounds on a baking sheet and broil them until they're golden brown (about 1 1/2 minutes per side).
Pulse the walnuts, Parmesan, and salt in a food processor until crumbly. Add the lemon juice. While the machine is running, add the oil in a slow, steady stream.
Spread the mixture on the rounds. Garnish the bruschetta with additional Parmesan, if desired.