Ham, Mushroom and Cheese Crêpes

Batter

• 300 g milk
• 200 g plain flour
• 20 g unsalted butter, diced
• 3 medium eggs
• ½ tsp fine sea salt
• 100 g water

Filling

• 60 g unsalted butter, diced, plus extra for frying
• 50 g plain flour
• 500 g milk
• ½ tsp fine sea salt, or to taste
• 1 pinch ground black pepper, or to taste
• 1 pinch ground nutmeg (optional)
• 100 g Gruyère cheese, cut in pieces
• 50 g shallots, halved
• 300 g button mushrooms, sliced
• 100 g ham, sliced, roughly torn in pieces