Heat oil in a medium pot and add pasta shells. Sauté the shells for a few minutes over medium heat then add minced onion. Cook until the shells are toasted and the onion is soft.
Pour chicken stock or broth, sea salt, black pepper, dried oregano, and crushed red pepper flakes over the pasta. Add one can of crushed tomatoes and stir to combine.
Add crushed garlic cloves and bring to a boil. Simmer over low heat for 10 to 15 minutes or until the shells are tender and the soup is reduced.
Notes
If you prefer your soup with more liquid, I recommend reducing the amount of shells you use instead of increasing the broth. Increasing the broth can make the soup taste watered down.
This soup is better enjoyed when you first make it. As it sits, the shells continue to absorb the broth so the soup gets quite condensed.
Store the leftovers in an air-tight container and melt some cheese into it when you reheat it and call it Mexican Mac n Cheese!