Put all the dressing ingredients in a screw-top jar and shake vigorously to mix, then set aside.
Put the almonds in a dry pan set over a medium heat and toast until they are golden. Scatter onto a plate and allow to cool, then roughly chop.
Trim the fennel, cut in half and slice as thinly as possible, then put in a large bowl. With a small sharp knife, slice off the top and bottom of the oranges. Cutting from the top to the base of the orange, remove the peel and white pith. Holding the orange over the bowl to catch the juice, carefully cut between the membrane to remove the orange segments and add to the fennel.
Put the red onion in a sieve and run cold water over them. Dry on kitchen paper before adding to the bowl.
Pour over half the dressing, toss well and arrange the fennel, orange and onion on a serving platter. Tear over the prosciutto and scatter the olives and toasted almonds on top, then drizzle with the remaining dressing if liked.