Drain pineapple, saving the juice. Set pineapple aside.
In a small bowl, whisk together pineapple juice, vinegar, soy sauce, agave, cornstarch, and red pepper flakes.
In a large skillet or wok on medium high, sauté garlic and onion in the olive oil, heating until onions are soft, about 3 minutes.
Add broccoli, diced pineapple, and cabbage, black pepper, ginger, garlic salt, stirring constantly. Cook for 2 minutes or until the veggies are warmed through.
Reduce the heat to medium and add the pineapple juice mixture, stirring constantly. Heat until sauce has thickened.