Heat oil in a skillet over medium heat. Saute onions until tender and translucent. Stir in mushrooms, and saute until soft. Add rosemary, and cook for 1 minute. Stir in cashews, and cook for 1 minute. Transfer to a medium saucepan.
Pour in chicken stock, and stir in rice. Cover, and bring to a boil. Reduce heat, and simmer until water is absorbed. Remove from heat, and let stand for 5 minutes. Stuff into the cavity of a small roasting chicken.