Blanch the broad beans in boiling water. Drain and refresh in ice-cold water.
Meanwhile, crush 2 garlic cloves with a little salt. Drain the broad beans and whizz in a food processor with the crushed garlic, lemon juice, mint and 1 tablespoon of oil to a course paste. Season.
Toast the bread on both sides. Cut the remaining garlic clove in half and rub one side of the toast. Drizzle with the remaining oil and leave to cool.
Cut the slices of toast in half and spread with goat’s cheese and a spoonful of the broad bean mixture. Top with lemon zest and black pepper and prosciutto (if using). You can also serve the broad bean mix and the goat’s cheese in separate bowls, along with the garlic toasts, for people to make up their own crostini.