Add millet and water to a large saucepan. Bring to a boil, reduce to simmer, cover, and let cook for 15 minutes. Most of the water should be absorbed, remove from heat and allow to sit for 2-3 minutes to fully absorb.
While the millet is cooking, combine the lemon juice, olive oil, garlic powder in a small bowl. Stir with a fork to combine. Add salt and pepper to taste.
Place millet in a large bowl. Add parsley, mint, tomato, and red onion. Add dressing and toss to coat. Season with additional salt and pepper to taste. Allow tabbouleh to chill and marinate in the refrigerator for about 30 minutes before serving.