Heat oven to 200C/180C fan/ gas 6. Tip the oil into an ovenproof casserole dish. Add bacon and fry for 2 mins. Add the leeks and cook until soft, but not coloured, for about 4-5 mins. Tip in rice and cook for 1 min more. Pour over stock. Cover and place in the oven for 20 mins, stirring halfway.
When rice is just tender and all liquid is absorbed, remove from oven and stir in peas. Place back in oven for 2 mins more. Remove and stir in cheese. Add zest and season.