Meanwhile, combine the tomatoes, onions, butter, oregano and salt into a saucepan. Bring to a simmer and cook uncovered for 10 minutes.
Add the mascarpone to the sauce, and stir. Continue to cook for 5 minutes.
Add the gnocchi to the boiling water. Once it floats to the top, remove the gnocchi and set aside in a bowl. Discard the water from the pan and add the gnocchi back into the pan. Add the pecorino and chopped arugula to the gnocchi. Stir gently to combine
Top with the creamy tomato sauce. Stir and serve warm.
Spinach can also be used to substitute the arugula. Be generous with the amount of arugula added to the gnocchi. Parmesan can also be substituted for the Pecorino Romano. Toasted pine nuts are a delicious topper for this meal.