Coming late to the party, it has to be salt & vinegar on chips. That said I remember when growing up we always had white vinegar at our caravan at Haltwhistle. Anyway, I digress. When it comes to bacon sandwiches (bread should always be toasted, or else it should be a stottie, or else Morton's rolls) I prefer ketchup and never brown sauce. Brown sauce is fine with sausage sandwiches (and again the bread should be toasted). Brie and bacon baguettes are allowed. There's a bar here in Las Vegas that does excellent poutine too. If in Brussels then Maison Antoine in Place Jourdan is supposedly the best place for frites and they also do a wide range of sauces. That said, there's a very good friterie outside Arlon station too!