Half fill a large pan with water. Add a pinch of salt. Cover and put on a high heat to come to the boil.
Heat your oven to 200°C/Fan 180°C/Gas 6. Add 4 tbsp rapeseed or olive oil to a large roasting tin or two medium tins. Put them in the oven to heat the fat.
Scrub and peel the potatoes. 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 8 mins.
While the potatoes simmer, peel the parsnips. Halve them. Slice the narrow ends in half, Cut the fat tops into quarters.
Drain the potatoes, then tip them back into the pan. Tip in the polenta. Put the lid on the pan and shake it a few time to coat the spuds in the polenta.
Take the roasting tin(s) out of the oven. Add the spuds and the parsnips. Season with a pinch of salt. Turn them a few times to coat in the hot oil. Roast for 45 mins till they're golden brown. If you're using two tins, swap them over half way through cooking.
While the veg roast, finely chop the rosemary leaves. When the spuds and 'snips are ready, transfer them to a serving bowl and scatter with the chopped rosemary to serve.