Pierce steak all over with a fork. Season generously with salt, pepper and garlic powder and set aside about 10 minutes at room temperature.
In a large bowl, combine onions, olive oil, balsamic, salt and pepper. Let onions sit a few minutes with the salt and balsamic to mellow a bit. Combine with tomatoes and fresh herbs and adjust seasoning if needed.
Heat grill or broiler on high heat. Cook steak about 7 minutes on each side for medium rare or longer to taste. Remove from grill and let it rest on a plate for about 5 minutes before slicing.
Slice steak think on the diagonal; top with tomatoes and serve.
by The Savvy Kitchen