Photograph: Mikkel Vang
Preheat the oven to 200°C, fan 180°C, gas 6. Place the bread slices on two baking trays, drizzle with olive oil and season with sea salt and freshly ground black pepper. Bake for 10-12 minutes, until golden brown. Transfer to a serving platter and leave to cool.
For the white beans, put the 2 tbsp olive oil, garlic and lemon zest in a saucepan and place over a low heat. Cook for 2-3 minutes, until the garlic is softened but not coloured. Add the beans, stock, a pinch of salt and plenty of black pepper. Warm through for 3-4 minutes, tossing in the flavoured oil until hot. Remove from the heat and lightly mash to roughly crush the beans (the bean mash will thicken as it cools). Tip into a serving bowl and leave to cool. Drizzle over extra-virgin olive oil and sprinkle generously with black pepper.
For the salsa verde, stir all the ingredients together in a bowl to combine and season with pepper only, to taste. The texture should be quite coarse. Transfer to a serving bowl.
When ready to serve, place the bread, white beans, prosciutto, parmesan and salsa verde on a platter on the table for everyone to assemble their own crostini.