Bring the beef stock and butter to a boil in a medium saucepan.
Stir in the salt, pepper and rice.
Turn off the heat, cover the pan, and allow the rice to absorb the liquid.
Once the liquid is absorbed (approximately 7-10 minutes), return the pan to medium/low heat and stir in the beans, corn, salsa, cumin, jalapenos and the frozen spinach.
Stir to combine and heat through.
Serve
Notes
Adding cooked beef or chicken is a great way to use up leftovers and turn this from a side dish to a 15 minute main course.