Wash and dry the potatoes and halve them lengthwise. With a knife, cut a cross about 1 cm deep on the cut surfaces.
Wash and dry the rosemary. Remove the leaves from the stalks.
Place the potatoes on a baking tray lined with baking paper with the cut surfaces facing upwards. Drizzle with 1 tbsp olive oil and sprinkle with the rosemary and some sea salt . Bake in a preheated oven at 180°C (160°C fan, gas 4).
Meanwhile, peel and finely chop the garlic. Peel and dice the onions.
Clean the mushrooms with a brush or kitchen paper and, depending on the size, cut into quarters or halves.
Heat the remaining olive oil in a non-stick pan and saute the onions and garlic over a low heat for 4-5 minutes.
Add the mushrooms, stir and season with salt. Cover and cook gently for 8-10 minutes. Remove the lid and simmer for 5 minutes until the liquid evaporates.
Wash and roughly chop the chives.
Mix the sour cream and two thirds of the chives with the mushrooms. Season with salt and pepper, sprinkle with the remaining chives and serve with the baked potatoes.