Because this sauce is so simple, it's important to use good-quality Italian tomatoes.
In a large skillet, heat olive oil over medium heat. Add garlic and cook slowly until golden brown on all sides, about 10 minutes.
Add tomatoes and their juice (do this carefully to keep oil from spattering), red-pepper flakes, and salt and pepper to taste; raise heat and bring to a boil. Reduce heat to medium and simmer, breaking up tomatoes into small pieces with the side of a wooden spoon.
Cook until sauce thickens, about 25 minutes. Remove from heat, transfer to a serving dish, and garnish with parsley.