Place the flour in a bowl, making a well in the centre; add the egg and half of the milk. Whisk to a smooth batter. Mix in the remaining milk and half of the thyme. Leave to rest for 20 minutes.
Lightly grease a 20cm frying pan with a little of the oil, then pour in enough batter to cover the base. Cook for 45 seconds, flip over and cook for another 45 seconds on the other side until cooked through. Repeat using all the mixture to make 4 pancakes. Set aside and keep warm.
Meanwhile, in a large frying pan, heat ½ tablespoon of the oil, add the mushrooms and shallots and fry for 10 minutes. Remove from the pan and set aside.
Add the remaining oil to the pan, along with the butternut squash and the sweet potato, and fry for 5 minutes until golden. Add the stock, the remaining thyme and the garlic, then cover and cook for 7-8 minutes, until the vegetables are just tender. Stir through the mushrooms and crème fraîche, then season and heat through.
To serve, place a pancake on each plate; spoon a generous amount of filling onto each. Roll up, cut in half on the diagonal and drizzle over a little of the sauce.