In a small bowl, whisk together soy sauce, oyster sauce, and cornstarch. Set aside.
Heat vegetable oil in a large skillet or wok over medium-high heat.
Add garlic and onion to the skillet and cook until fragrant and onions are translucent.
Add turkey slices to the skillet and cook until browned on all sides.
Add bell peppers to the skillet and stir-fry for 2-3 minutes, until slightly softened.
Pour the sauce mixture over the turkey and bell peppers. Stir well to coat everything evenly.
Continue cooking for another 2-3 minutes, until sauce has thickened and turkey is cooked through.
Season with salt and pepper to taste.
Serve the stir-fry hot with steamed rice or noodles.