Bring a large pot of lightly salted water to a boil for the bucatini.
Heat the olive oil in a skillet over medium-low heat, add the finely minced onion and cook until translucent, about 4–5 minutes.
Add the canned tomatoes with their juices to the skillet and gently crush them with a wooden spoon; season with most of the basil leaves (reserve a few for finishing).
Cover and simmer the sauce on low heat for about 15 minutes, stirring occasionally, until creamy but still slightly chunky; taste and season with sea salt and black pepper.
Meanwhile, cook the bucatini in the boiling water according to package directions for al dente, draining 2–3 minutes before the suggested time if you prefer firmer pasta; drain in a colander.
Remove the sauce from the heat, stir in the remaining fresh basil leaves, then add the drained bucatini to the skillet and toss to combine; drizzle a little extra-virgin olive oil if desired and serve immediately.
