Prepare all ingredients ahead of time (onion, garlic, rapini, tomatoes, parmesan). This will make for easier cooking!
Fill a large pot with water and heat on high until it comes to a boil.
In a large saucepan, add oil and heat on low-medium.
Once oil is hot, add onions and cook for 1-2 minutes, until they start to become translucent.
Next, add garlic, anchovy, salt, pepper and red chili flakes.
Add rapini and cook for 2 minutes or until it begins to soften.
Add tomatoes and cook for 5-6 minutes. Stir every few minutes so that they don't burn. Once cooked, squish slightly with a back of a spoon.
Once water boils, add spaghetti in and cook until al dente (mine was about 9 minutes).
Add butter and lemon juice to sauce, allowing butter to melt.
Fill a small mug with pasta water and add to tomato sauce. Once mixed in, the sauce should become thicker and glossy. If that's not the case add a bit more pasta water. If it seems too watery, bring up the heat and allow some of the liquid to cook off.
Once pasta is done, drain and add to sauce. Remove from heat.
Add parmesan and mix immediately and quickly, so the cheese doesn't become stringy.