Pre-heat your oven to 200°C/395°F (fan-forced).
Boil the potatoes until starting to soften (almost cooked through). Depending upon the size of your potatoes, this will take about 10-20 mins.
Drain the potatoes well.
Place 2 tbsp of the olive oil into a large baking tray. Add the potatoes and with the back of a fork, partially crush them.
Scatter over the carrots, rosemary and garlic.
Drizzle with the remaining olive oil and season with salt and black pepper.
Bake for 20 minutes then add the prosciutto and cook for a further 15 mins.
Defrost the peas by sitting them in boiling water. Drain well.
Once the potatoes and prosciutto are crispy, remove from the oven and scatter over the peas.