Wash the potatoes thoroughly, cover and cook in water for about 20 minutes.
For the crust, finely crumble the bread in a large bowl. Wash the thyme, shake dry and remove the leaves. Melt 75 g butter in a pan. Roast the breadcrumbs and thyme leaves for about 3 minutes, turning them over.
Season with pepper. Put it back in the bowl. Finely grate the parmesan. Mix half the parmesan and the whole mustard into the crumbs.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Drain potatoes, quench briefly and cut in half lengthwise. Place potatoes on a baking tray with the cut surface facing upwards.
Season with salt. Spread the crumb mix evenly on top. Sprinkle with the rest of the Parmesan, spread 2 tablespoons of butter in flakes on top. Bake in a hot oven for 12-15 minutes. Take out and arrange. Serve with salad.