Heat the olive oil in a large pot over medium heat until shimmering. Add the chopped onion and sauté, stirring occasionally, until it becomes translucent and fragrant, about 5 minutes.
Add the minced garlic to the pot and cook for another minute, until fragrant and slightly golden, stirring constantly to prevent burning.
Pour in the crushed tomatoes along with their juices, stirring to combine with the onion and garlic mixture. Bring the sauce to a gentle simmer and cook for 10 minutes to develop flavor.
Stir in the broth and season with dried basil, red pepper flakes (if using), salt, and pepper. Bring the mixture to a boil over medium-high heat.
Add the dry pasta shells to the boiling soup, stirring to prevent sticking. Reduce the heat slightly and cook uncovered until the pasta is tender, about 8-10 minutes.
Once the pasta is cooked and the soup has slightly thickened, stir in the shredded mozzarella cheese until melted and creamy.
Sprinkle the grated Parmesan cheese over the soup and give it a gentle stir to distribute the cheesy goodness evenly.
Taste the soup and adjust the seasoning with more salt and pepper if needed. Allow it to simmer for another minute to meld all flavors.
Remove the pot from heat and ladle the steaming, cheesy soup into bowls. Garnish with additional Parmesan or fresh herbs if desired.
Serve hot, enjoying the comforting layers of tomato, cheese, and pasta in each spoonful.
