1. Set the oven to 180 degrees fan.
2. Cut the fennel bulb in half down the top of the stems, then into 1.5cm chunks, cutting out the tough core. Peel off the outer layer from the leeks, then cut them in half lengthways, then into three sections horizontally. Wash them thoroughly as leeks like to hide dirt!
3. Peel and slice the onion into 'rainbows'
4. Lightly grease a dish or pan with approx diameter 22cm, then arrange the fennel, leeks and onion evenly across the pan. Dot with knobs of butter.
5. Scatter in the bay leaves, pour over the cream and milk, season well then cover with foil and bake for 45 minutes.
6. Whilst the gratin is cooking, prepare the topping. Melt the butter in a pan and finely chop the herbs. Once the butter has melted, combine it with the other ingredients.
7. Take the gratin out of the oven and remove the foil, making sure that the vegetables have started to turn brown and caramelise. Turn the oven up to 200 degrees, scatter on the crumb, then put back in the oven for a further 10 minutes until crispy, bubbling and delicious.
8. Allow to cook for 5 minutes before eating with good crusty bread or serving alongside a hearty roast.
Leek & Fennel Gratin
4 as a side or 3, for lunch
For the gratin:
1 bulb of fennel
2 medium leeks
1/2 an onion
200ml double cream
150ml full fat milk
3 bay leaves
A little butter
For the crunchy topping:
25g herbs of your choice (I mix parsley & thyme)
10g finely grated parmesan cheese
The zest of one lemon