Heat oil in a large nonstick skillet over medium-high heat. Add onion and garlic; cook, stirring frequently, until onion is tender, about 3 to 5 minutes.
Add quinoa to skillet; cook, stirring frequently, until quinoa starts to turn golden brown, about 2 minutes.
Add broth, black pepper and red pepper flakes to skillet; bring to a boil. Cover skillet and reduce heat to low and simmer for 13 minutes; stir in spinach. Cover skillet and cook until spinach and quinoa are tender and liquid is absorbed, about 3 to 5 minutes; season with salt. Yields about 1/2 cup per serving.