Half fill a large pan with water. Add a pinch of salt. Cover and put on a high heat to come to the boil. Scrub and peel the potatoes (see our tips on how to use up your potato peelings). Chop them into roughly equal-sized chunks, around 3-4cm across. When the water is boiling, add the potatoes. Bring to a gentle simmer, and then simmer for 5-6 mins.
Heat your oven to 200°C/Fan 180°C/Gas 6. Add 1 tbsp olive oil to a large roasting tin or 2 medium tins. Put them in the oven to heat the oil for 10 mins.
Drain the potatoes, then tip them back into the pan. Put the lid on the pan and shake it gently a few time to rough the edges of the potatoes (be careful not to break the potatos up too much).
Take the roasting tin(s) out of the oven. Add 50g of the butter and swirl to melt it. Add the spuds. Season with a pinch of salt. Turn them a few times to coat in the hot oil and butter. Roast for 45 mins till they're golden brown. If you're using 2 tins, swap them over half way through cooking.
While the veg roast, finely chop the rosemary leaves. When the spuds are ready, transfer them to a serving bowl and scatter with the chopped rosemary to serve.