Heat a large thin-based frying pan over a medium heat. Add the oil and, once hot, add the onions and a pinch of salt. Cook for 5-6 minutes, or until the onions are soft and translucent, then add the garlic and chilli flakes and cook for a further two minutes.
Stir through the tinned tomatoes, breaking up any large bits with the back of a wooden spoon. Add the rice and a pinch of salt and pepper and stir well to mix. Pour over the stock and leave the pan for seven minutes without stirring. Turn the heat down to medium-low if the liquid looks like it is evaporating too quickly.
Scatter over the mussels and the peas - do not mix through (you are trying to create a crisp bottom to the rice). Cover with a lid and cook for a further five minutes, or until the liquid is all absorbed, the rice is tender but still with a very slight bite, and the mussels and peas are hot all the way through.
Serve the paella hot, making sure everyone has a bit of the crunchy rice stuck to the bottom of the pan.