Heat the oil and 1 tbsp of the butter in a flameproof casserole. When the butter is melted, add the chopped onion and sauté gently until lightly golden. Meanwhile, heat the broth to boiling in a saucepan.
Add the tomato paste to the casserole and stir thoroughly until it is completely incorporated. Add the rice and sauté for 3 minutes, stirring constantly with wooden spoon, then pour in 1/2 cup of the boiling broth and keep stirring very gently until the rice has absorbed all the broth (about 3 minutes). Taste for salt and pepper, then continue to add hot broth, little by little, stirring continuously until all the broth is absorbed and the rice is al dente (about 18 minutes in all).
Remove the casserole from the flame, add the remaining butter, and mix thoroughly. Transfer the risotto to a serving dish and serve immediately.