Preheat the oven to gas 6, 200°C, fan 180°C. Cut the French baton into 12 slices and arrange on 2 baking trays. Brush each slice with the basil-infused oil and bake for 15 minutes. (You can do this stage the day before and simply keep the cooled toasts in an airtight container.)
Peel the avocados and remove the central stone. Place in a food processor with the mayonnaise and cooked peas, and blitz.
To assemble the crostini, lay a thin slice of mozzarella on a toast followed by a slice of tomato, a spoonful of the avocado mixture and a folded ‘wave’ of Parma ham.
To finish, top each toast with a fresh basil leaf and secure through the centre with a cocktail stick decorated with a fun flag, if you wish. Repeat with each of the remaining toasts.