Boil a large pot of salted water. Add rice and stir well. Simmer for 12 to 15 minutes or until rice is tender. Drain in a sieve or fine-mesh strainer. Transfer to a large rimmed baking sheet and cool completely in the fridge. Once cool, transfer to a container and keep refrigerated for up to 24 hours.
Melt butter in a large frying pan over medium heat. Add onion and cook for 4 to 6 minutes or until soft and translucent. Add cold rice and cook, stirring often, until piping hot, 4 to 7 minutes. Stir in dill. Serve immediately.