Preheat oven to 375°F. Cut the leaves and stem off the cauliflower so it sits flat. Place in a large baking dish and surround with tomatoes, onion wedges, and green beans. Season veggies with salt and coat with olive oil. Meanwhile, in a small saucepan over medium heat, whisk together balsamic and brown sugar. Bring to a boil, then immediately reduce to a simmer. Let simmer until reduced by half, about 15 minutes. Brush glaze all over cauliflower, reserving some for basting. Roast until golden and tender, 1 hour, periodically basting with glaze. Garnish with parsley and serve.