2 baguettes - day-old is best; halved lengthwise = 4 pieces
500 g white button mushrooms (pieczarki) - finely chopped
1 onion - finely diced
2 tbsp butter - or oil
250 g grated cheese - Gouda/Edam, or a Gouda-mozzarella mix
1/2 tsp dried marjoram
Salt and black pepper - to taste
Ketchup and chopped chives - to finish