Cook the rice in plenty of well salted boiling water, then drain. Set aside and keep warm.
In a large frying pan over a medium/low flame, gently fry the onion in the olive oil and half the butter until starting to soften. Add the pumpkin and a pinch of salt, stir so every cube glistens, then add 100ml water.
Cover with a lid and cook for 15 minutes, or until the pumpkin is soft and there is just a little liquid.Add the chestnuts to the pan, stir and cook for a few minutes, then add the rice and stir again. Taste and salt and pepper to taste.
In another pan, melt the butter and fry the sage leaves for 30 seconds, then pour both over the rice, and serve immediately.