Heat the olive oil in a 4-quart saucepan over medium high heat. Add the onion and cook, stirring frequently, for 2 to 3 minutes or until the onions are crisp-tender. Stir in the garlic and saffron threads and cook, stirring frequently, for 1 minute.
Add the rice and cook, stirring constantly until crisp. Add the chicken broth and salt, cover with a lid, and cook for another 7 to 11 minutes or until the rice is fluffy and the liquid is absorbed. Cover and set aside.
Heat the canola oil in 12-inch non-stick skillet. Add the plantains and fry for 4 to 6 minutes, turning several times, until light brown and tender.
In a small bowl, mix the sour cream and milk, stirring constantly, for 1 or 2 minutes. Place the rice on a platter. Top with the plantains and swirls of the sour cream mixture. Serve with the remaining sour cream mixtures on the side.