Polish croquettes with mushrooms and sauerkraut

Crêpe mixture

• 330 g water
• 230 g full cream milk
• 2 eggs
• 20 g grapeseed oil
• 310 g plain flour
• 1 pinch sea salt

Filling

• 500 g sauerkraut
• 1000 g water
• 1 tbsp oil, plus extra for frying
• 1 onion (approx. 150 g), cut into halves
• 500 g button mushrooms, cut into halves
• 1 ½ tsp sea salt, to taste
• 1 - 1 ½ tsp ground black pepper, to taste
• 120 g breadcrumbs
• 2 tbsp butter, for frying