Preheat the oven to 200C fan/gas mark 6. Break the eggs into a mixing bowl and whisk them lightly to combine yolks and whites. Stir in the mustard and 100ml of the milk.
Using a breadknife, cut the croissants in half horizontally then dunk them in the egg and milk, pushing them down so they are thoroughly soaked. Lay the bottom halves of each croissant side by side in a baking dish or roasting tin.
Grate the fontina. Divide the speck between the croissants, tearing it as necessary, then sprinkle 25g of cheese over each of the ham-covered halves. Place the top halves back in place on each croissant, then scatter over any remaining cheese.
Bake in the preheated oven for 15 minutes till the cheese has melted.