Make the sauce: Heat 1/2 tsp oil in a small saucepan over medium heat; add minced garlic and minced ginger and sauté 30 seconds.
Remove the pan from heat and stir in 1/4 cup chicken broth, rice vinegar, honey, brown sugar, tomato paste, soy sauce, and oyster sauce; season lightly with salt and pepper.
Whisk the remaining 2 Tbsp chicken broth with 1 Tbsp cornstarch in a small bowl, then return the mixture to the saucepan and bring to a boil, whisking constantly, until thickened, about 1 minute; remove from heat and set aside.
Prepare the chicken: In a mixing bowl whisk together 1/4 cup cornstarch, 2 large egg whites, 1/2 tsp salt, and 1/4 tsp pepper until slightly frothy; add diced chicken and toss to coat evenly.
Heat 1 Tbsp oil in a large non-stick sauté pan over medium-high heat. Add half the coated chicken in a single layer and cook, turning occasionally, until cooked through and golden on the outside, about 7 minutes; transfer to a plate.
Add the remaining 1 Tbsp oil and cook the remaining chicken the same way, then return all cooked chicken to the plate.
While the chicken cooks, heat 2 tsp oil in a large skillet over medium-high heat; add diced bell peppers and onion and sauté until tender, about 6–8 minutes, adding pineapple during the last minute.
Add the cooked chicken and vegetable–pineapple mixture to the sauté pan (or combine in the large skillet), pour the prepared sauce over everything, and toss to coat evenly and heat through.
Serve warm over white or brown rice.
