Heat oven to 200 °C. Then heat 1 ½ tsp. oil over medium heat, in a 10 "nonstick skillet that can be used in the oven. Add onion and garlic. Cook, stirring occasionally, until the onion is smooth and translucent, about 10 minutes. Transfer to a medium bowl, clean skillet and set aside.
Add the spinach, quinoa and cream cheese in a bowl, along with the onion mixture and stir until the ingredients are well distributed. Separately, in a medium bowl and using a fork; Mix the eggs, the cheddar cheese and the marinade. Pour the egg mixture over the Quinoa mixture and stir until everything is well combined.
Heat the remaining oil in the same pan over medium heat. Pour the quinoa mixture into the pan and press with a spatula to form an even layer. Cook until the bottom of the quinoa is browned, and lastly transfer the pan to the oven. Cook until the quiche is well cooked (to check insert a thin knife in the center of the quiche, it should come out clean) and the top of the Quinoa turns golden brown, for about 15 to 20 minutes.
Carefully invert the quiche onto a serving plate and cut into portions. Serve hot or at room temperature