Cut the croissants in half lengthwise. If I they are still soft inside, put them on a plate and let them dry out more (overnight if necessary).
In a bowl, whisk together the egg, milk, vanilla and cinnamon.
Dunk each croissant half into the mixture so that it's fully coated. Set them aside on a plate.
Oil lightly and heat a large non-stick frying pan over medium heat.
Add the croissant halves, cut-side facing downwards.
After 3 minutes or so they should be golden brown, so flip them over and cook for another 2 minutes or so on the other side.
Remove from the pan and serve with chopped nuts and honey or maple syrup.