Wash the fish fillet under running water, pat dry and acidify with the lemon juice.
Skin and slice the tomatoes. Heat the oil and fry the onions cut into rings. Pat the fish dry and brush with the mustard, cut into strips about 3 cm wide and layer alternately with the tomato slices and onion rings in a buttered casserole dish, seasoning each layer with salt, pepper and thyme. At the end, sprinkle with the grated cheese and bake at 200 ° C for 25-30 minutes.