Heat the olive oil in a large, heavy-based saucepan. Add the onion and reduce the heat to medium-low. Cover and sweat for 10 minutes, stirring occasionally until translucent and soft. Remove the lid and stir in the garlic, continuing to cook for a minute or so.
Add the rice, oregano and bay leaf. Stir until the rice is coated in oil and then add a ladleful of stock or water. Stir until the liquid has been absorbed, then add another ladleful. Continue in this way until you've used about 800ml of the water. This will take 15-20 minutes. You will get bored of stirring but it's really important to continue doing so, so make sure you have the radio on or someone to chat to! Taste the rice. It should be soft but with a tiny bit of bite. If it's not cooked add a further ladleful of stock or water.
Stir in the fresh mint, parmesan and butter (if using) followed by the peas. Stir over a low heat for a minute or two until the peas have warmed through. Remove the bay leaf.
Take out a portion for the children and/or baby and then season well with salt and pepper. Alternatively allow people to season their own at the table.