In a medium size pot, melt butter over medium heat.
Add the onions and sauté until soft.
Add the garlic and continue to sauté for another minute.
Pour the rice into the pot and stir to coat the rice with the buttered onions and garlic.
Allow the rice to toast in the pot for a few minutes. Some grains, but not all, will develop a slight brown color, this is what you want to see.
Add the salt, cumin, and coriander.
Pour the chicken broth and tomato sauce into the rice and bring to a boil.
Reduce heat to low and simmer, covered, for 15 minutes, or until all the liquid is absorbed.
When rice is fully cooked, stir in black beans and serve hot.