Step 1: Preheat the oven to 400ºF. Oil a 2-quart gratin or baking dish.
Step 2: Heat a medium frying pan or a wide saucepan over medium-high heat.
Step 3: Wash the spinach and without spinning dry, add to the pan and wilt in the liquid left on the leaves after washing.
Step 4: You may have to do this in 2 batches.
Step 5: As soon as the spinach wilts, remove from the heat and rinse with cold water. Squeeze dry and chop. Set aside.
Step 6: Wipe the pan dry and heat 1 tablespoon of the olive oil in it over medium heat.
Step 7: Add the onion and cook, stirring often, until tender, about 5 minutes.
Step 8: Add the garlic and stir with the onion until fragrant, about 1 minute.
Step 9: 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.
Step 10: 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.
Step 11: Scrape into the gratin dish. Sprinkle the Parmesan over the top and drizzle on the remaining tablespoon of olive oil.
Step 12: 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.