In a medium saucepan set over medium heat, heat the oil until shimmering, about 2 minutes. Add the onion and saute until browned, 5 to 7 minutes. Add the garlic and saute for 2 minutes more. Add the stock and quinoa and mix until fully combined. Add the cumin, cayenne, and some salt and pepper.
Increase the heat to high and bring the mixture to a boil. Immediately reduce the heat to low, cover, and simmer for 15 to 20 minutes, or until all the liquid is absorbed. Add the spinach and gently toss with the quinoa until wilted.
Remove the pan from the heat. Stir in the corn and black beans. Crumble the feta over the top and garnish with cilantro.