Blanching the peppers before stuffing them helps hold their shape and retain their bright color. We like to use a variety of colors so the finished dish is extra colorful. Slideshow: More Quinoa Recipes
Rinse the quinoa in 3 changes of water in a bowl, rubbing the grains and letting them settle each time before pouring off the water (if the quinoa does not settle, drain in a large fine-mesh sieve after each rinse).
Cook the quinoa in a 3- to 4-quart saucepan of boiling well-salted water until tender, about 15 minutes. Drain well in a sieve.
Meanwhile, heat 2 tablespoons of the olive oil in a medium skillet over moderate heat. Add the onions and cook, stirring occasionally, until golden and softened, 8 to 10 minutes. Add the chorizo and cook until it starts to release its oil, about 2 minutes. Stir in the mushrooms and cook, stirring occasionally, until golden, about 3 minutes.
Add the drained quinoa and stir to combine. Season to taste with salt and pepper. Remove from the heat, and stir in the ricotta salata and chopped parsley. Set aside.
Cut the bell peppers in half lengthwise and discard the ribs and seeds. Cook the peppers in a medium pot of simmering salted water until crisp-tender when pierced with a knife, about 5 minutes. Drain in a colander and let cool slightly.
Preheat the oven to 400°. Lightly oil a medium baking dish. Fill the pepper halves with the quinoa mixture and arrange in the dish. Drizzle the peppers with the remaining 1 tablespoon olive oil. Bake until quinoa mixture is golden-brown on top and piping hot, about 20 minutes.