Bring the stock to a gentle simmer with the saffron.
Heat the oil and butter in a saucepan over moderate heat. Add the onion, garlic and rice and cook for 3 to 4 minutes, stirring with a wooden spoon.
Add 1 ladleful stock to the rice, stirring for about 3 minutes until the rice has absorbed almost all of the liquid.
Continue adding the stock, 1 ladleful at a time, until it has all been absorbed (about 20 to 25 minutes) and the rice is cooked to a creamy al dente stage. (You may need to add a little extra hot water.)
Remove from the heat and stir in the lemon juice and lemon zest. Season to taste with salt and pepper. Return the saucepan to the heat for 1 minute, then stir the risotto again before serving. Top each serving with shaved Parmesan cheese and another good grinding of pepper, if liked.