Add the diced onion to the pot and sauté until soft, about 3-4 minutes.
Stir in the garlic and cook for another minute, until fragrant.
Pour in the broth, marinara sauce, and diced tomatoes with their juice.
Stir in the dried basil, oregano, and crushed red pepper flakes (if using). Season with salt and pepper to taste.
Bring the soup to a boil, then reduce the heat and let it simmer for about 20-25 minutes to allow the flavors to meld together.