Place the flank steak in a large sealable plastic bag. Mix together water, cornstarch, sugar, soy sauce, and garlic in a medium bowl. Pour the marinade over the steak and seal the bag tightly. Turn the bag so that it is completely sealed, place it on a plate, and refrigerate overnight.
Preheat oven to 375 degrees F. Line a large baking sheet with foil or parchment paper. Remove steak from marinade and discard the excess marinade. Place the steak on the prepared baking sheet and bake for 25 minutes, or until cooked through.
Meanwhile, steam the broccoli florets in a large saucepan over medium-high heat for about four minutes, or until tender. Serve steak and broccoli florets together.