Rough tear the bread into pieces, place in a food processor and make into breadcrumbs. Put the breadcrumbs into a bowl with the herbs and mix together. Season with salt and add a squeeze of lemon juice. Stir in enough of the oil and melted butter to make the mixture moist.
Meanwhile, put the plaice fillets, skin side down, on a sheet of greased foil on a grill rack. Season to taste, dot with butter and spread with the herb mixture. Place the fillets under a preheated moderate grill and cook for 10 minutes until the fish are done.
Arrange the plaice fillets on a bed of onions and tomatoes or other vegetables and serve at once.