In a large frying pan, heat the olive oil over medium heat. Add the onion and garlic and stir until they are fragrant and transparent, about 30-60 seconds. Add the celery and continue to cook while stirring for an additional 2-3 minutes, until the celery softens slightly. Add the mushrooms and cook, stirring, until they are tender, about 10 minutes. Add in soy sauce then stir the parsley and cook for an additional 1 minute. Remove from the heat and stir in the cooked quinoa. Season with salt and pepper to taste, place in a serving dish, and garnish with the extra parsley before serving.