Preheat the oven to 200ºC, fan 180ºC, gas 6. Heat the oil in a pan and stir in the flour. Cook for 1 minute before whisking in the milk, a little at a time. Bring to the boil and simmer for 2 minutes before stirring in 3/4 of the Cheddar, 3/4 of the parmesan and the mustard.
Meanwhile, cook the parsnips and carrots in a large pan of boiling water for 8 minutes until just tender, adding 1 of the sliced leeks for the last 3 minutes. Drain and tip into a 1-litre baking dish. Scatter over the beetroot wedges then cover with the cheese sauce.
Sprinkle the breadcrumbs over the gratin, along with the rosemary and the remaining leek and cheeses. Bake in the oven for 15-20 minutes, until golden and bubbling.