Heat the oil in a skillet over medium heat, stir in onion and sausage and cook for 5 minutes until slightly brown. Add the rice and stir until well coated. Pour in the chicken stock, bring to a boil, and reduce to a simmer.
Cover and cook until the rice is tender and the liquid has been absorbed, about 20 minutes.
Put the rice in a large bowl, stir in the remaining ingredients and mix well. Season to taste with salt and pepper. Place the rice mixture in a well-oiled casserole dish, cover and bake at 350° for 30 minutes.