Heat the butter and oil in a small pan. Add the garlic and herbs, leave to infuse on the switched-off hob for approx. 15 mins.
Cook the potatoes in salted water for approx. 15 mins., drain and allow the water to evaporate for approx. 5 mins. Place the potatoes in a bowl, pour the herb oil into the bowl through a sieve, set aside the herbs. Mix the potatoes with the oil and salt, crushing the surface of the potatoes slightly in the process (see note). Transfer the potatoes to a baking tray lined with baking paper.
Approx. 45 mins. in the centre of an oven preheated to 200°C. Turn the potatoes twice during cooking. Scatter the reserved herbs over the potatoes, return to the oven for approx. 15 mins.
Mix the crème fraîche with the mayonnaise, mustard and honey, then season. Serve the dip with the roast potatoes.