Preheat the oven to 350F. Line a 6 or 12-cup muffin tin with 6 parchment muffin cups or grease well with coconut oil.
Add the quinoa to a small pot with ⅓ cup of water. Bring to a boil, then reduce heat to low and simmer, covered, for 15 minutes. Transfer to a mixing bowl and let it cool for a few minutes.
Meanwhile, in a large skillet, warm the oil over medium heat. Add the mushrooms and saute until softened, about 5 minutes. Stir in the spinach, garlic and salt and cook for 1 more minute, or until the spinach has wilted. Remove from heat and stir in the feta, if using.
Add the egg, oregano and salt to the quinoa and mix thoroughly to combine. Diving the mixture in heaping tablespoons between the 6 muffin cups.
Divide the veggie mixture in an even layer on top of the quinoa, leaving room for the egg on top. Crack an egg on top of each one. Sprinkle each egg with some salt, if you’d like.
Bake for 20-25 minutes, or until the eggs are set and cooked to your liking. Allow to cool before storing in an airtight container in the fridge. To reheat, pop a couple into the microwave until warmed through, about 15 seconds.