Bring broth + 1/2 teaspoon salt to boil in medium saucepan; add quinoa. Cover, reduce heat to low, + simmer until quinoa is tender + broth is absorbed, about 15 minutes. Remove from heat; fluff with fork. Cover + reserve.
Meanwhile, heat a large non-stick skillet to medium. Add olive oil + garlic; sauté about 1 minute, until garlic is fragrant. Add bell peppers, green beans, zucchini + squash. Season with salt + black pepper to taste. Sauté until just tender, about 8 minutes.
Add cooked quinoa, thyme, lemon juice, + green onions to vegetables in skillet; toss to combine. Season with sea salt + pepper, + garnish with parsley, if using.