When is it appropriate to cook pasta directly in the sauce, and when is it not?
Cooking pasta directly in the sauce only works if the sauce has the right consistency and cooking time, while finishing it in the sauce almost always improves the result.
If the sauce is too thick, it doesn't have enough water for the pasta to cook properly. You can thin it out, but add water gradually and keep a close eye on it so it doesn't evaporate before the pasta is ready. You also need a pan or pot large enough to submerge all the pasta. If you don't have one, it's best to cook the pasta separately to avoid uneven cooking, or choose a shorter pasta shape. Furthermore, if the sauce doesn't need to simmer for 10 to 14 minutes, there's no point in cooking the pasta in it.
Some sauces don't require as much cooking time as pasta: pesto, cacio e pepe, and carbonara are a few examples. On the other hand, there are pasta dishes designed to cook in their own sauce, such as pasta e ceci or one-pot pasta. In these cases, the sauce is prepared during cooking and initially contains much more water than a conventional sauce, since the pasta needs a significant amount of water to cook.
Even diluted, pasta takes longer to cook in sauce than in water. After numerous tests, Daniel Gritzer, senior culinary director at Serious Eats, discovered that pasta removed from boiling water after six minutes and finished in sauce took four minutes longer than pasta left in boiling water. A one-pot spaghetti recipe, for example, uses up to 6 cups of broth, reserving 2 to adjust the consistency, and cooks the spaghetti in 15 minutes instead of the usual 8 to 10.
If the sauce is already ready, has the perfect consistency, and you're in a hurry to serve, finishing the pasta in it is a good solution. If you can't cook the pasta in the sauce from the start, it's best to at least finish it there. The goal is to integrate both ingredients: the starch in the pasta and its cooking water help the sauce adhere to the noodles. Without this step, the sauce won't stick to the pasta unless you rub it in very vigorously.
To achieve this, place a little sauce in a pan over low heat and add the pasta a few minutes before it's al dente. It's best to thin the sauce with a ladleful of the pasta cooking water, which is usually salty and starchy, stirring occasionally to prevent the pasta from sticking to the bottom and adding more water if needed. Keep tasting the sauce: if it's too salty, it's best to use tap water. The pasta should finish cooking in the sauce until it's al dente.
