Start by rinsing the quinoa under cold water using a fine-mesh strainer. This removes any bitterness from the quinoa's natural coating.
Heat 1 tablespoon of olive oil in a medium saucepan over medium heat. Add the quinoa and toast for 2-3 minutes, stirring frequently, until it smells nutty.
Pour in the vegetable broth, bring to a boil, then reduce heat to low. Cover and simmer for 15-20 minutes, or until the liquid is absorbed and quinoa is tender.
While the quinoa cooks, heat the remaining olive oil in a large skillet over medium-high heat. Add the onions and cook until translucent, about 3-4 minutes.
Add the mushrooms and garlic to the skillet. Cook for 5-6 minutes, or until the mushrooms release their moisture and begin to brown.
Stir in the soy sauce, thyme, salt, and pepper. Cook for another minute to combine the flavors.
Add the fresh spinach to the mushroom mixture and cook just until wilted, about 2 minutes.
Fluff the cooked quinoa with a fork and divide it among serving bowls. Top with the mushroom-spinach mixture.
If desired, garnish with toasted pine nuts and crumbled feta cheese before serving.