In a large skillet or pot, heat the olive oil over medium heat. Add the diced red onion and sauté for about 3-4 minutes, or until it becomes translucent. Incorporate the minced garlic and continue to sauté for an additional 1-2 minutes until fragrant.
Stir in the rice and quinoa medley, ensuring it’s well-coated with the oil and aromatics. Pour in the water, and bring the mixture to a boil. Once boiling, reduce the heat to low, cover, and simmer according to the package instructions, typically around 15-20 minutes, or until the grains are tender and the water is absorbed.
Once the grains are cooked, remove the lid and fluff with a fork. Mix in the drained black beans, corn, diced tomatoes, yellow bell pepper, and serrano chile (if using). Stir gently to combine all the ingredients.
Sprinkle in the cumin, kosher salt, and black pepper. Taste and adjust the seasoning as needed. Add freshly squeezed lime juice and minced cilantro, folding them into the mixture until everything is well incorporated.
Serve the Mexican Rice, Beans, and Quinoa Medley warm, garnished with additional cilantro and lime wedges, if desired. This dish pairs wonderfully with grilled meats, tacos, or can stand alone as a satisfying meal.