Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Cover, and bring the water to a boil over high heat. Add the broccoli and steam until just tender, 2 to 6 minutes depending on the size of the broccoli florets. Remove the broccoli to a serving bowl and toss with butter, add salt and pepper to taste.
Heat the tuna, teriyaki sauce and garlic powder in a frypan over medium heat. Cook and stir until the tuna has heated through, about 5 minutes. Pour the tuna over the broccoli and serve.