Preheat your oven to 425°F (220°C) and line a large baking sheet with parchment paper. Pat the chicken cubes completely dry with paper towels to remove all moisture, then place them in a large bowl. Add the egg, 3 tablespoons of cornstarch, salt, and pepper to the chicken, mixing thoroughly with your hands or a spoon until every piece is evenly coated with the mixture.

Arrange the coated chicken pieces in a single layer on the prepared baking sheet, making sure they don't touch each other. Bake for 18-20 minutes, flipping halfway through, until the chicken is golden brown and cooked through.

While the chicken bakes, prepare the sweet and sour sauce by whisking together the vinegar, sugar-free ketchup, soy sauce, 1 tablespoon of cornstarch, Splenda packets, garlic powder, cumin, salt, and pepper in a medium bowl until completely smooth with no lumps.

Heat the olive oil in a large skillet or wok over medium-high heat. Add the diced onion and red bell pepper, sautéing for 4-5 minutes until the vegetables are slightly softened and have some golden edges.

Pour the prepared sauce mixture into the skillet with the vegetables and cook, stirring constantly, for 2-3 minutes until the sauce thickens and becomes glossy. The sauce should coat the back of a spoon when ready.

Add the baked chicken pieces to the skillet with the thickened sauce and vegetables, tossing gently to ensure every piece is well coated with the sweet and sour sauce. Cook for an additional 1-2 minutes just to heat everything through, then serve immediately.