To make the dressing - place all ingredients into a bowl and whisk well to combine. Set aside and make the salad.
Place slices of prosciutto (if using) on a baking sheet lined with baking paper. Place the walnuts on another sheet also lined with baking paper. Put both trays in the oven. Bake the walnuts for ~6-7 mins. Bake the prosciutto for the full 10 minutes or until golden. Set both aside to cool. (See note 1 for this step)
While the prosciutto and walnuts are cooking, assemble the salad. The best way to do this is layer the salad (see note 2 below). Add 1/3 of the arugula/rocket, 1/3 peach slices, and 1/3 goat's cheese to a large bowl. Crush over 1/3 of the cooled walnuts and repeat this twice more so you have three layers.
Crumble the cooked prosciutto if using over the top of the salad.
Drizzle 2/3 of the dressing over the top and serve straight away. More dressing can be added to each bowl served if desired.
Notes
Note 1: Let your senses guide you with the walnuts and prosciutto! Every oven is different, and they're being cooked at high heat. If the walnuts smell and look cooked in your oven well before the 7 min mark take them out. You don't want them to burn. Same goes for the prosciutto. It's done when crispy and may need more or less than 10 mins depending on how hot your oven runs.Note 2: I layer this salad so it doesn't need to be tossed and looks beautiful when served. You should have three layers of arugula/rocket, peaches, goat's cheese, and walnuts when done.Note 3: The dressing is generous so if any is left, keep it in a jar with a lid on in the fridge. It can be used within the week for another salad.