In a food processor, coarsely grind the onions. You can alternatively mince them with a knife.
Heat the oil in a wide skillet over medium heat. Add the onion and saute until very dark, about 5 minutes.
Drain the rice and place it in a pot with 8 cups water. Add the chestnuts, turmeric, and saffron. Bring the water to a boil.
Reduce the heat to medium-low, cover, and cook the rice for about 1 hour. Remove from the heat and let sit for 5 minutes.
Add the reserved onions to the rice and stir to combine. Season with salt and pepper.
Notes
you might find vacuum-packed steamed chestnuts. In this case, reduce the water to 6 cups, omit the dry chestnuts, add 12 ounces total vacuum-packed chestnuts towards the end of the cooking.
Recipe by Go Dairy Free at https://www.godairyfree.org/?p=166574