Add the oil to a sauté pan or wok. Heat on high heat and add the chicken and half of the garlic. Do not disturb the chicken and let it cook on high heat for about 3 minutes, or until browned.

Stir the chicken, rotating it to the other side to brown. Add in the diced onions. Cook them together stirring occasionally until the onions are softened and the chicken is cooked through.

Meanwhile, Put the cornstarch, ginger, red pepper flakes, and remaining half of the garlic into a small bowl. Gradually whisk in the soy, broth, vinegar, and brown sugar.

Add the sauce to the pan and cook for 2 minutes, or until the sauce is thickened.

Toss the diced pineapple and chopped bell pepper into the chicken and sauce. Stir it until they are coated with sauce.

Top it with scallions. Stir and cook for another minute or two. If you want the bell peppers softer you can toss them in with the chicken while it is cooking or cook the entire mixture for longer.