Heat olive oil in a large skillet over medium heat. Stir quinoa and onion into hot oil and cook until onion is translucent, about 5 minutes. Add half the minced garlic to the quinoa mixture and cook until garlic is fragrant and slightly softened, 1-2 more minutes. Add salt and broth to the quinoa mixture. Bring to a boil, then reduce heat to medium-low, and simmer until the liquid has been absorbed, about 15 minutes.
In a food processor, finely pulse the remaining minced garlic, cilantro, scallions, jalapeño, spinach leaves, and lime juice. Add mixture to quinoa after the quinoa has been cooking for 15 minutes. Blend thoroughly and cook, covered, for 5 minutes. Remove from heat and let stand 5 minutes. Fluff with a fork and serve. Top with tomatoes, if desired, for garnish.
Notes
You can substitute rice for the quinoa, if desired, and just cook the rice as directed on the package.