Preheat the oven to 220C/gas 7/fan 200C. Pour the oil into a large roasting tin or ovenproof dish and put in the oven for a couple of minutes to heat through.
Add all the vegetables to the tin, except the beans, stirring to coat them in the hot oil. Season with salt and pepper and return to the oven for 15 minutes until beginning to brown.
While the vegetables are roasting, warm a pan with some oil and gently fry the spice blend, chilli, and mustard seeds.
Add the stock and saffron and bring to the boil.
Mix the rice and green beans with the vegetables in the tin, then pour over the stock mixture. Lower the oven to 190C/gas 5/fan 190C. Cover the dish tightly with foil and bake for 30 minutes until the rice is tender and the liquid has been absorbed. Stir in the lemon juice and check the seasoning.