-
Boil well-salted water for pasta. Cook to al dente, and then drain, reserving ½ cup of the pasta water.
-
Heat 2 tbsp olive oil in skillet. When oil is warm, add diced onions and let cook for 5-7 minutes, or until translucent.
-
Add minced garlic, red pepper flakes, and tomatoes, and cook for an additional 5 minutes or so. The tomatoes should be softened and release juices in the skillet.
-
Add pasta water to tomato and onion mixture, and bring to a simmer. Stir in the pasta and butter, and top with arugula. Serve with parmesan cheese, if desired.