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First, cook the prosciutto. Heat a non-stick pan, add the slices in a single layer without oil and cook for a few mins each side until they are starting to crisp.
Meanwhile, stone the avocado and scoop the flesh into a bowl. Roughly mash, then spread on the bread. Top two slices of the bread with the tomato, some prosciutto, the lettuce and the rest of the prosciutto. Sandwich with the remaining bread and halve to serve. You can keep the sandwich together with a cocktail stick.