Place the potatoes in a large pan of cold, well-salted water with a couple sprigs rosemary. Bring to a simmer and cook until the potatoes are tender, then drain
Chop the leaves of the remaining rosemary finely. Mix with the garlic and 2 tbsp oil in a large bowl
Preheat oven to 190°C fan. When cool enough to handle, cut the potatoes in half lengthways and toss in the oil mixture. Roast on a metal baking sheet, cut side down until nicely golden
Meanwhile, place the large pan over a medium heat with a little oil. Add the leeks and sweat until softened. Stir in the creme fraiche, parsley and mustards. Warm through and season to taste
Spoon the leek mixture onto a serving plate and top with the roasted potatoes and a little chopped parsley
