Dice the chicken breast into bite-sized pieces and place them in a bowl. Toss with sea salt, black pepper, and cornstarch until each piece is evenly coated.
In a small mixing bowl or jar, whisk together the tomato puree, rice vinegar, tamari, honey, minced ginger, and minced garlic to create the stir-fry sauce.
Heat the avocado oil in a large non-stick skillet or wok over medium-high heat until it begins to shimmer.
Add the chicken pieces to the hot skillet in a single layer and cook for 5-6 minutes, turning occasionally, until the exterior is golden-brown and crispy.
Add the sliced bell peppers and red onion to the skillet. Sauté for 3-4 minutes until the vegetables are tender-crisp and the colors remain vibrant.
Pour the prepared sauce over the chicken and vegetables, then stir in the pineapple chunks.
Continue to cook for 1-2 minutes, tossing constantly, until the sauce thickens into a glossy glaze that coats all ingredients thoroughly.
Remove from heat and serve immediately while the chicken maintains its signature crunch.