Heat the butter in a large skillet over medium-low heat. Stir in the onions with a big pinch of salt and pepper. Cook, stirring often, for 25 to 35 minutes, until the onions are golden and caramely.
Towards the end of the onions caramelizing, bring a pot of salted water to a boil and cook the pasta according to the package directions.
Add the garlic and cherry tomatoes into the skillet with the onions. Cook, stirring often, until the tomatoes begin to burst, about 10 minutes.
Stir the pasta into the skillet. Stir in the basil ribbons and the parmesan cheese. Taste and season with a pinch of salt and pepper if you feel it needs it.
Serve immediately with extra basil and parmesan for topping.