Heat the oil in a large skillet over medium heat and add the garlic and onion. Cook until the onion becomes translucent, about 2-3 minutes.
Toss in the peppers and mushrooms, and cook until the water has been released and evaporated, about 5 minutes.
Add the broccoli, asparagus tips and ginger, and cook for about 3 minutes more.
Add the beef and cook until heated through, about 3 minutes.
Pour in the teriyaki sauce and stir until heated through.
Whisk together the soy sauce and cornstarch and pour into the pan. Cook for 2 minutes more.
Divide the brown rice among 4 plates. Top with equal portions of the stir fry, garnish with the scallions and serve immediately.
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