Combine all ingredients for sweet and sour sauce in a small bowl and whisk until corn starch has dissolved.
In a small sauce pan heat sauce over medium heat. Stir frequently until thickened and glossy, about 10 minutes. Add a pinch of red pepper flakes, if desired.
Heat oil in a skillet over medium heat. Add bell pepper, carrots, and garlic and cook until desired tenderness.
Steam cauliflower to desired tenderness and add to skillet. Add pineapple and cook for 2-3 minutes.
Pour desired amount of sauce over veggies and toss to coat. Serve with rice, optional.