Heat the oven to 200°C fan / 220°C conventional / Gas 7. Put the fat or oil into a large roasting tin and heat in the oven for 10 minutes.
Peel the potatoes and cut into generously sized, evenly sized pieces. Place in a saucepan, cover with cold water and add a good pinch of salt.
Bring to the boil, then simmer for 8–10 minutes, until the edges are tender but the centres still hold their shape.
Drain thoroughly and leave to steam-dry for 5 minutes. Shake gently in the saucepan to fluff up the edges. Sprinkle with the semolina, if using.
Carefully lower the potatoes into the hot fat—it may spit. Turn to coat and arrange in a single layer with space between them. Season with salt.
Roast for 50–65 minutes, turning after about 25 minutes. Add the garlic and herbs for the final 20 minutes so they flavour the potatoes without burning.
Serve immediately, with a final sprinkle of salt and freshly ground black pepper.