Place the flour in a large bowl and make a well in the centre. Beat together the egg, egg yolk and milk and pour into the well. Beat with a wooden spoon until smooth. Rest for 30 minutes.
Meanwhile, heat the sunflower oil and butter in a large frying pan and add the garlic. Cook for just a few second,s then tip in the mushrooms and cook for a few minutes until starting to turn golden and soften. Add the leeks and cook for a further 5 minutes until softened and tender.
Stir in the spinach leaves and cook until wilted into the mushrooms and leeks. Stir in the crème fraiche and remove from the heat.
To make up the pancakes, rub a 20cm frying pan with a little sunflower oil and heat over a medium heat. Add a small ladleful of batter, just enough to coat the base of the pan, then swirl around the pan quickly until it covers the base of the pan. Cook for a minute until lightly coloured underneath, then using a palette knife flip over and cook for a further minute. Tip out onto a plate and place the pan back on the hob. Make 9 more pancakes.
Preheat the oven to 200°C, fan 180°C, gas 6. Lay a pancake on a board and spoon a few tablespoons of the filling in the centre. Add a tablespoon of grated cheese and roll up. Place in a large shallow baking dish and repeat with the other pancakes, putting them side by side in a single layer. Sprinkle over the remaining cheese and bake for 20 minutes until golden and bubbling. Adults could add a slice of cooked smoked ham to the pancake before adding the filling and rolling up. For weaning babies, purée the pancake filling without the cheese.