Place the quinoa and vegetable broth in a medium saucepan. Bring to a boil, then reduce heat to low, cover, and simmer for 15 minutes until the liquid is absorbed. Remove from heat and let it sit covered for 5 minutes.
Heat the olive oil in a large skillet over medium heat. Add the minced garlic and cook for 1 minute until fragrant.
Add the sliced mushrooms to the skillet. Cook for 8 to 10 minutes, stirring occasionally, until the mushrooms are browned and most of their liquid has evaporated.
Stir the fresh spinach into the skillet. Cook for 2 to 3 minutes until the spinach wilts. Season with salt and black pepper.
Fluff the quinoa with a fork and stir in the lemon juice. Add the quinoa to the skillet and mix everything together until well combined.
Divide the mixture into four bowls and top each with crumbled feta cheese before serving.