Preheat oven to 180ºC/350ºF/Gas Mark 4. Grab a large pan and heat some of the oil and butter. Add the lamb cubes and leeks and cook for 2-3 minutes.
Add the rice and give it a good stir. Then add the hot stock, stir well and bring to the boil. Pour the mixture into an ovenproof dish, cover with foil and place in the oven for 20 minutes.
Next, add the courgettes and green beans, and stir well. Return to the oven, covered, for about 10 minutes.
Remove from oven, add a little more stock if needed for the desired consistency. Stir through the herbs, peas and cheese.