Add the butter to a medium sauce pan or a deep frying pan. Bring the heat to medium high and, when the butter is melted, add the rice and stir, toasting it for 3 minutes.
Add the cumin, salsa, and water to the saucepan, and bring it to a boil.
Cover the pan and reduce the heat to low, cooking at a simmer for 20 minutes. After 20 minutes, stir the rice and cook it uncovered 5-10 more minutes, or until all of the liquid is absorbed,
It's now ready to serve — top with red onion, cilantro, and jalapeños, if desired.