Preheat the oven to 220°C.
Place the potatoes in a pan of cold salted water and bring to the boil.
Reduce the heat, cover and simmer for 6-8 minutes, until just softened.
Drain and return to the pan for a minute or two to dry out. Put the lid back on the potatoes and shake vigorously to break up and soften the edges. Set aside until needed.
In a second saucepan, bring salted water to the boil. Add the cauliflower and simmer for 4-5 minutes.
Remove from the heat and drain. Keep aside for a minute.
Preheat a roasting tin with 4 tbsp duck fat for a minute or so (until just smoking).
Carefully tip the potatoes into the hot oil, basting the potatoes with the hot fat.
Add the cauliflower pieces to the tray with the garlic and rosemary.
Season well and sprinkle with grated cheese.
Place the roasting tin in the oven and roast for 20-25 minutes until crispy around the edges and golden brown.
Remove from the oven and serve immediately with an extra sprinkle of cheese.