Heat 2 tablespoons oil in a large saucepot over medium-high heat. Add the rice and cook, toasting until the rice begins to smell nutty, about 5 minutes. Add the chicken stock along with some salt and bring it to a boil, then reduce to a simmer and cover. Simmer until the rice is tender and has absorbed all the liquid, about 25 minutes
Meanwhile, heat the remaining tablespoon oil along with the butter in a large skillet over medium-high heat. Add the onion, pepper and a generous pinch of salt and cook until just tender and beginning to brown, about 5 minutes. Add the garlic, sausage, thyme and sage, and cook, breaking up with a wooden spoon until browned and cooked through, 4 to 5 minutes more.
When the rice is cooked and liquid is absorbed, turn off the heat, remove the lid and add the sausage and vegetables. Fluff and stir gently with a fork to combine. Add the rice to a serving dish and sprinkle with the chives before serving hot.