Cook rice according to package instructions. Meanwhile, in a wok or large skillet, heat oil over high. Add garlic and cook until fragrant, 10 seconds. Add broccoli, cauliflower, bell pepper, and pinch of red pepper flakes and cook, stirring, about 4 minutes. Add broth and stir to combine. Cook, stirring constantly, until vegetables are crisp-tender, about 3 minutes. Add oyster sauce and cook, stirring to combine, 1 minute. Spoon over rice and serve immediately.