Preheat the oven to 180°C / 350°F / Gas Mark 4. Trim the leeks and slice into 1cm chunks. Heat the oil and garlic in a frying pan and add the leek slices, and cook over a medium heat until they are starting to soften and just browning at the edges.
Chop the kale, removing any woody stalks, add to the pan and cook for a further 2 minutes. Finely chop the rosemary, and pick the leaves from the thyme (discarding the stalks). Add the white wine to the pan, bring to the boil and cook for 2-3 minutes until the liquid has reduced, then add the cream, rosemary and thyme, and cook for a further 2-3 minutes. Season well with salt and black pepper, then remove from the heat and set aside to cool.
Unroll the filo pastry sheets, and slice them into long triangles as pictured.
Take a ring tin (or savarin mould) roughly 24cm in diameter, and spray with cooking spray or grease with dairy-free margarine. Layer the filo triangles in the tin, with the pointy ends overlapping the centre, making sure that each triangle overlaps the last by at least 1cm (see picture). Keep spraying the pastry with cooking spray (or brush with melted margarine) as you go to make sure it doesn't dry out.
When your pastry is 3 layers thick, spoon in the filling, then fold over the edges to cover it completely, and spray/brush again with oil/margarine.
Bake the strudel for 20 minutes, until the pastry is golden brown. Then remove from the oven, place a large baking tray on top and flip over, the remove the tin to expose the underside. Spray or brush the top with oil/margarine and scatter with poppy seeds, then return the strudel to the oven for a further 5 minutes until golden brown and crispy.