In a small bowl, whisk together the rice vinegar, soy sauce, ketchup, sugar, and cornstarch until the cornstarch is dissolved. Set aside.
In a large pot, bring the vegetable or chicken broth to a simmer over medium heat.
If using protein, heat the vegetable oil in a separate pan over medium heat and cook the protein until browned and cooked through. Set aside.
Add the sliced bell peppers, carrots, mushrooms, pineapple chunks, and onions to the pot with the simmering broth. Cook for about 5 minutes or until the vegetables are tender.
Stir in the prepared sweet and sour sauce mixture. Continue to cook for another 2-3 minutes until the soup thickens slightly.
If using cooked protein, add it to the pot and simmer for an additional 2-3 minutes to heat through.
Serve the sweet and sour soup hot, garnished with chopped green onions.
Enjoy your homemade sweet and sour soup!Feel free to adjust the ingredients and proportions to suit your taste preferences. You can also add other vegetables or protein options of your choice. Serve the soup as an appetizer or as a main course with steamed rice for a complete meal.