Preheat oven to 375 degrees F.
To make the pesto, combine the peas, basil, garlic, lemon zest, lemon juice, pistachios, olive oil, salt and pepper in a food processor and pulse until completely smooth. It'll take a few minutes. Taste and adjust seasonings accordingly. Set aside.
In a small bowl, combine the ricotta, garlic, lemon juice, and pinch of salt and pepper until smooth.
Lay the baguette slices in a single layer on a baking sheet. Drizzle with about 2 tablespoons of olive oil and toast in the oven until lightly golden brown. To assemble the crostini, spread each toasted bread slice with pea pesto, ricotta, a piece of prosciutto, and a few walnuts. Drizzle them with olive oil and top with coarse salt and black pepper. Enjoy!