Preheat the oven to 400ºF. Oil a 2-quart gratin or baking dish.
Heat a medium frying pan or a wide saucepan over medium-high heat.
Wash the spinach and without spinning dry, add to the pan and wilt in the liquid left on the leaves after washing.
You may have to do this in 2 batches.
As soon as the spinach wilts, remove from the heat and rinse with cold water. Squeeze dry and chop. Set aside.
Wipe the pan dry and heat 1 tablespoon of the olive oil in it over medium heat.
Add the onion and cook, stirring often, until tender, about 5 minutes.
Add the garlic and stir with the onion until fragrant, about 1 minute.
Stir in the spinach and season with salt and pepper. Remove from the heat.
Beat the eggs in a large bowl and add 1/2 teaspoon salt.
Stir in the quinoa, the onion and spinach mixture, the Gruyère/cheddar, the chorizo and the sage. Add freshly ground pepper and stir the mixture together.
Scrape into the gratin dish. Sprinkle the Parmesan over the top and drizzle on the remaining tablespoon of olive oil.
Place in the oven and bake until nicely browned on top, about 25 minutes.
Remove from the heat, allow to sit for about 5 minutes, and serve.