Bring a large pot of salted water to a boil. Add the spaghetti and cook according to the package instructions until al dente. Drain and set aside.
Heat vegetable oil in a large skillet or frying pan over medium heat. Add the chopped onion, green bell pepper, and red bell pepper. Sauté until the vegetables are tender, about 5 minutes.
Add the minced garlic and cook for another minute, until fragrant.
Add the ground beef or pork to the skillet. Cook, breaking it apart with a wooden spoon, until browned and cooked through. Drain any excess fat if necessary.
Stir in the tomato paste and cook for 2 minutes.
Add the tomato sauce, diced tomatoes, and beef or chicken broth. Stir well to combine.
Season with dried thyme, oregano, paprika, black pepper, salt, and crushed red pepper flakes (if using).
Mix in the soy sauce and vinegar. Reduce the heat to low and let the sauce simmer for 10-15 minutes, stirring occasionally, until it thickens slightly and the flavors meld together.
Add the cooked spaghetti to the skillet with the sauce. Toss well to coat the pasta evenly with the sauce.
Garnish with chopped fresh parsley and scallions if desired. Serve hot.