Cut the chicken breast into bite-sized cubes and season evenly with sea salt and black pepper.
In a small glass bowl, whisk together the coconut aminos, rice vinegar, tomato paste, minced ginger, and minced garlic to create the sauce.
Heat the avocado oil in a large skillet or wok over medium-high heat until shimmering.
Add the chicken cubes to the skillet and cook for 5-6 minutes, stirring occasionally, until golden brown and cooked through.
Remove the chicken from the skillet and set aside on a plate.
In the same skillet, add the diced red and green bell peppers along with the red onion, sautéing for 3-4 minutes until slightly softened.
Stir in the fresh pineapple chunks and pour the prepared sauce over the vegetables.
Bring the sauce to a light simmer for 2 minutes, then return the chicken to the skillet, tossing well to coat everything in the glaze.
Serve the sweet and sour chicken immediately over the warm cooked brown rice.