To start, prep all the ingredients, including the chicken thighs, bell peppers, scallions, garlic, and onions. You will want to make sure all the ingredients are ready to go into the skillet as stir-fries cook quickly. Dice the boneless chicken thighs into 1” pieces with a sharp knife, place into a bowl, and set aside. Mince the garlic and set it aside. Cut the sides off of the bell peppers, leaving the membrane and seeds on the center part. Cut each bell pepper slice on a diagonal to make triangular pieces.
Cut an onion in half and peel off the skin. Cut each half into quarters and break them apart.
Preheat a large nonstick skillet or wok over high heat. Make sure the skillet or wok is very hot. Then add in the oil and swirl the pan. Add in the chicken and allow the chicken to cook undisturbed for several minutes until the bottoms are browned very nicely. While the chicken is browning, you can add in ½ tsp kosher salt and ½ tsp black pepper. Stir the chicken now and finish cooking it all the way through.
Add in the minced garlic and give the chicken a toss. Cook for 30 seconds so the garlic can become fragrant. Place the chicken in a separate bowl and set aside.
Add in ½ tbsp of oil and swirl the pan. Add in the onions and cook for 1 minute, stirring constantly. Then add in the bell peppers. Cook the bell peppers until they are just beginning to soften. They should still be reasonably crunchy (remember they will continue to cook in the next step)—Season the onions and peppers with ¼ tsp kosher salt.
Add the chicken back into the pan with the bell peppers, giving the entire mixture a good toss to distribute all the ingredients evenly—taste adding more salt and pepper if desired.
Salt and pepper chicken should be salty and peppery! Top the finished dish with sliced scallions.