For the crust, wash the thyme, shake dry and remove the leaves. Coarsely grate the rolls. Mix with thyme and fried onions. Melt 50 g butter in a pan. Fry the breadcrumb mixture in it until golden brown. Season with salt and pepper and take out.
For the vegetables, separate the tender inner leaves from the kohlrabi and set aside. Peel, wash and cut the kohlrabi into sticks. Cook in 1⁄8 l boiling salt water for about 10 minutes. Add the frozen peas after about 7 minutes cooking time and cook them. Drain the vegetables and collect the cooking water.
For the fish, wash the fillet, dab dry and cut into 8 portion pieces. Season with salt. Heat oil in a pan. Fry the fillets for about 1 minute on each side. Put them on a baking tray, spread with mustard and spread the crumb mixture evenly on top. Bake under the hot oven grill for 5-8 minutes.
For the sauce, heat 2 tablespoons of butter in a saucepan. Sweat flour in it while stirring. Deglaze with milk and vegetable boiling water, bring to the boil while stirring and simmer for about 5 minutes. Season with salt, pepper and nutmeg. Cut kohlrabi leaves into strips. Heat the leaves and vegetables in the sauce again. Serve fish with the vegetables. Mashed potatoes taste good with it.