Heat 3 to 4 tablespoons olive oil in a large deep skillet over medium-high heat.
Add the diced red onion and cook, stirring occasionally, until softened and just beginning to lightly caramelize, about 5 to 7 minutes.
Stir in the minced garlic and cook for about 1 minute, until fragrant.
Add the dry rice and quinoa medley and toast, stirring, for about 30 seconds.
Pour in 3 cups water (or the amount the grain package recommends for 1 cup dry grains). Reduce heat to low, cover, and cook until water is absorbed and grains are tender, about 10 to 15 minutes; check at 10 minutes and stir, cooking longer if needed.
If desired, drizzle 1 to 2 additional tablespoons olive oil while cooking to prevent sticking.
When the grains are tender, add the drained black beans, drained corn, diced tomatoes, diced bell pepper, and optional serrano; stir to combine.
Stir in the ground cumin, then add the kosher salt and black pepper; taste and adjust seasonings or heat as desired.
Fold in the minced cilantro and squeeze in lime juice to taste, stir to combine, and serve warm, at room temperature, or chilled.