Wash the meat and dab dry. For the crust, whisk mustard and egg with a fork. Wash parsley, shake dry and chop finely. Wash thyme, shake dry and pluck leaves from the stalks. Peel and wash potatoes, dab dry and grate. Mix potatoes, breadcrumbs, herbs, parmesan and turmeric. Stir in egg-mustard mixture. Season to taste with salt and pepper
Place the meat side by side on a baking tray lined with baking paper. Spread the crust evenly over the meat with your hands (it sticks a bit). Press down lightly all around. Bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 25 minutes.
In the meantime peel and finely dice the onion. Wash the tomatoes, possibly halve them depending on size. Heat oil in a pan, add tomatoes and onion and braise at low to medium heat for about 5 minutes. Season with salt and pepper
Take the meat out of the oven, cut it open and arrange it with the tomatoes. Garnish with thyme. Serve with baguette