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1
: Adjust oven rack to middle position and preheat oven to 350u00b0F. Place squash cut-side down on a rimmed baking sheet and add water until water level rises 1/4-inch up the sides of the pan. Bake squash until fork-tender and peeling away from the skin, about 35 minutes. Remove squash from oven, let cool slightly, then use a fork to scrape the inside of the squash, pulling away the spaghetti-like strands from the peel. Place squash flesh in a large bowl and cover to keep warm.
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2
: While the squash bakes, combine walnuts, garlic, and lemon zest in the bowl of a food processor. Pulse until mixture is coarsely ground. Add parsley and pulse to combine. With machine running, stream in 3/4 cup olive oil or more, as needed, until pesto is sufficiently loose. Add Parmesan, pulse to combine, and season to taste with salt and pepper. Set aside.
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3
: Place baguette slices on a baking sheet and drizzle with 2 tablespoons olive oil. Bake until lightly browned and crisp, about 8 minutes. Remove from oven and let cool.
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4
Add 1/2 cup pesto to warm squash and toss to combine, adding more pesto as necessary to coat squash. Season to taste with salt and pepper. Heap about 1 tablespoon squash on each crostini, then top each crostini with some torn prosciutto, dividing prosciutto evenly between 12 crostini. Serve immediately.
