1. Cook quinoa: In a saucepan, bring water to a boil. Add quinoa, reduce heat to low, cover, and simmer for 15 minutes or until water is absorbed. Fluff with a fork.
2. Roast mushrooms: Preheat oven to 400°F (200°C). Place sliced portobello mushrooms on a baking sheet, drizzle with olive oil, and season with salt and pepper. Roast for 20 minutes.
3. Combine ingredients: In a large mixing bowl, combine cooked quinoa, roasted mushrooms, red bell pepper, cucumber, feta cheese, and parsley.
4. Dress salad: Drizzle with lemon juice and additional olive oil if desired. Toss to combine. Season with salt and pepper to taste.