Place saffron threads in a small mixing bowl along with the boiling hot water. Mix and let soak for 10 to 15 minutes.
Place rice in a bowl, cover with very hot water, and let soak for 5 to 10 minutes. Drain, rinse with cold water, and drain thoroughly.
Heat the oil in a 3 to 4 quart pan over medium-high heat. (If you are using achiote seeds instead of saffron, just let a teaspoon of them cook in the oil for 2 to 3 minutes before adding the rice.) Once the oil is hot, add the dried rice and saute for 1 to 2 minutes. Incorporate the onion, tomato, and garlic, stir, and continue to cook until the rice changes color to a milky white. It should sound and feel heavier, as if you were moving sand in the pot, about 4 to 5 more minutes.
Pour in the chicken broth, saffron mix, and salt and stir everything together. When the liquid starts to boil, cover the pot, lower the heat to low and continue cooking for about 20 more minutes, or until the rice is cooked through and the liquid has been mostly absorbed.
If the grains don't seem soft and cooked through, add a bit more chicken stock or water and let it cook for another 5 minutes or so. Turn the heat off and let it sit covered for 5 to 10 minutes. Fluff with a fork and serve.
Rice can be made ahead of time and reheated later the same day. Before reheating, add 1 tablespoon of water and heat, covered over the lowest heat possible. Once it has cooled down, it can be kept in a closed container in the refrigerator for up to 2 days.