In a large skillet, heat the olive oil over medium heat. Once hot, add the diced red onion and cook for about 5 minutes, or until it becomes translucent. Add the minced garlic and sauté for an additional minute, until fragrant.
Stir in the rice and quinoa medley, mixing well to coat the grains in the oil and onion mixture. Toast the grains for about 2-3 minutes, stirring frequently.
Add 3 cups of water to the skillet (or the amount specified on the package for your specific grains). Bring to a gentle boil, then reduce the heat to low, cover, and let it simmer according to the package instructions, typically around 15-20 minutes.
Once the grains are cooked, stir in the black beans, corn, diced tomatoes, yellow bell pepper, and serrano chile (if using). Mix well to combine all ingredients.
Add cumin, kosher salt, black pepper, and half of the minced cilantro. Stir everything together and let it cook for an additional 5 minutes, allowing the flavors to meld.
Remove from heat and stir in the freshly squeezed lime juice and the remaining cilantro. Taste and adjust seasoning if needed, adding more lime juice if you prefer a tangy kick.
Transfer the Mexican Rice, Beans, and Quinoa Medley to a serving bowl, garnishing with extra cilantro and lime wedges if desired. Serve warm and enjoy!