For the plaice roulades with root vegetables, season the plaice fillets with salt and pepper, roll them up and place them in a well-buttered baking dish. Cook in the oven at 200 degrees for about 20 minutes.
For the mustard sauce, heat the butter. Cut the leek into thin slices and fry in the butter. Add flour and continue frying. Add milk in portions until the mixture becomes a thick sauce.
Remove the sauce from the heat, stir in mustard and season with salt and pepper. Root vegetables, cut into thin slices and place in a pot with butter, water and salt.
Steam the vegetables until cooked and add a little lemon juice.