Put a large roasting tray over a medium heat and either add a few generous lugs of olive oil or carefully spoon a little of the fat from the meat you’re cooking. Add the garlic and rosemary leaves. Put the vegetables into the tray with a good pinch of salt and pepper and stir them around to coat them in the flavours. Spread them out evenly into one layer – this is important, as you want them to roast, not steam as they will if you have them all on top of each other. Put them into the preheated oven for about 1 hour, or until golden, crisp and lovely. Serve immediately, with your roast dinner and your gravy.