In a bowl, whisk together the egg white, cornstarch, and ¼ tsp of the salt. Add the chicken pieces and stir to coat evenly. Let sit for 15 minutes at room temperature or up to overnight in the refrigerator.
Meanwhile, whisk together ¼ cup of water, ginger, vinegar, ketchup, brown sugar, remaining ¼ tsp of salt, and 1-2 pinches cayenne pepper (or more to taste--optional).
Set a large saute pan over medium-high heat. Pour in 1 tbsp of the oil and swirl to coat. Add the red and yellow bell pepper chunks and cook for 2 minutes. Remove onto a plate.
Return pan to the stove on medium-high heat. Pour in the remaining 1 tbsp of oil and swirl to coat (be sure to fully coat the pan so the meat won't stick). Add the chicken to the pan, spreading it out in one layer.
Let the chicken saute, untouched, for 1 minute, until the bottoms are browned. Turn the chicken and cook other side the same for 1 minute. The chicken will still be pinkish in the middle.
Add the cooked bell peppers, pineapple chunks, and sauce mixture to the pan. Let simmer for about 2 minutes until the chicken is cooked through. Remove from heat. Let the chicken rest for a few minutes till the sauce thickens.
Garnish with sliced scallions, if desired. Serve over rice or even over quinoa.