Add cornstarch to the second batch of marinade and whisk until smooth, creating a slightly thicker sauce.
Add oil to a large skillet and heat over medium heat. Add garlic and ginger and cook, stirring constantly, until fragrant, about 30 seconds. Add scallions and cook another minute.
Add bell pepper and snap peas, stirring often, until just tender, 2 to 3 minutes.
Pour in the sauce and cook, stirring, until thickened, about 30 seconds.
Add the roasted tofu and pineapple chunks and cook, stirring gently, until heated through, about 1-2 minutes more.
Serve over rice.