Combine the leek, carrots, parsnips and garlic in a large bowl, then stir in the beaten egg. Combine the flour, onion salt and a grinding of black pepper, then stir this into the veg mixture until fully combined.

Melt the butter in a non-stick frying pan over a medium heat. Spoon in the veg mixture and press down lightly using a spatula so it covers the base. Cook for 5-6 mins until the spatula slides easily underneath. Carefully invert onto a plate, then slide back into the pan and cook on the other side for 5-6 mins until golden all over. Scatter over the spring onions thyme leaves, and serve sliced into wedges.