Heat the oil in a large ovenproof Dutch oven. Add the onions and cook over medium-low heat for 5 minutes, stirring occasionally. Add the bell peppers and cook over medium heat for 5 more minutes. Lower the heat, add the garlic, and cook for 1 minute longer. Stir in the rice, chicken stock, saffron, red pepper flakes, salt, and pepper and bring to a boil. Cover the pot and place it in the oven. After 15 minutes, stir the rice gently with a wooden spoon, and return it to the over to bake uncovered for 10 to 15 more minutes, until the rice is fully cooked.