Heat a large, fairly deep skillet over medium-high heat. Add 3 to 4 tablespoons olive oil and the diced red onion. Cook, stirring occasionally, until the onion has softened and just begins to lightly caramelize, about 5 to 7 minutes.

Add the finely minced garlic and cook, stirring nearly constantly, until fragrant, about 1 minute.

Add the 1 cup dry rice and quinoa medley to the skillet and stir to combine with the onion mixture. Let the grains toast, stirring, for about 30 seconds.

Pour in 3 cups water (or the amount the grain package directs for 1 cup dry grains). If desired to help prevent sticking, drizzle an extra 1 to 2 tablespoons olive oil into the water.

Bring the mixture to a gentle simmer, then reduce the heat to low. Cover the skillet and cook until the water is absorbed and the rice and quinoa are tender, about 10 to 15 minutes. Check at 10 minutes, stir gently, and continue cooking as needed because grain cook times vary.

Once the grains are tender, add the drained and rinsed black beans, drained and rinsed corn, diced tomatoes, diced yellow bell pepper, and the optional serrano chile (use ½ to 1 serrano, seeded and minced very small; start with less if you prefer less heat). Stir to combine.

Add 1 to 2 tablespoons ground cumin (not teaspoons), 1 teaspoon kosher salt, and ½ teaspoon freshly ground black pepper. Stir well and taste; adjust seasoning or heat as desired. If the mixture seems dry, drizzle in more olive oil to your preference.

Stir in ⅓ cup finely minced fresh cilantro and 1 to 4 tablespoons freshly squeezed lime juice, adding the lime juice to taste. Mix to combine.

Serve immediately warm, or cool to room temperature or chill. Store any leftovers in an airtight container in the refrigerator for up to 5 days.