Preheat oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer). Line baking tray with baking paper. Cut bread into cubes of approx. 1.5 cm and spread on the baking tray. Roast in a hot oven for 10-15 minutes until golden brown, turning from time to time.
In the meantime clean and wash the leek and cut it into fine rings. Peel garlic and chop finely. Wash thyme, shake dry, pluck off leaves. Heat 50 g butter in a pan. Steam leek and garlic for 6-8 minutes at medium heat until soft. Stir in thyme and season with salt and pepper.
Turn down the oven temperature (electric cooker: 160 °C/circulating air: 140 °C/gas: see manufacturer) Lightly grease a box mould (25 cm long) and line it with baking paper. Coarsely grate the cheese. Mix crème fraîche, cream, eggs, egg yolk and mustard. Season with salt and pepper. Spread 4 tablespoons on the bottom of the mould. Approx. 1⁄3 Add the bread cubes. Ca. 1⁄3 Sprinkle cheese on top. Then add half leek, 1⁄3 bread cubes, 1⁄3 cheese, the rest leek and bread. Slowly pour the rest of the egg cream into the mould and press the bread cubes down slightly. Leave to soak for about 30 minutes.
Rest Sprinkle cheese over the pudding and bake in a hot oven 1 1⁄2-1 3⁄4 for hours until golden brown (stick test!), possibly cover with foil after 1 hour. Remove and let cool in the mould for about 10 minutes. Then turn out and serve hot or cold. Serve with ham and sliced very thinly