How To Make Pesto Stick To Pasta
Choose pasta that has a rough surface and make sure to use a little bit of the pasta water to emulsify and thicken the sauce.
When trying to make pesto (or any sauce for that matter) stick to pasta, the biggest gains are to be had from buying the right kind of pasta.
The dried stuff you find in the supermarket is made from a simple semolina and water dough which has been extruded through a die and shaped. Generally speaking, budget pasta will have been extruded through an aluminium or teflon die, resulting in a smooth surface texture. There's nothing wrong with that per se, but the premium pasta in your local deli will almost always have been extruded through a bronze die. This results in a rough texture which dramatically increases the surface area of the pasta and provides the gold standard of stickiness.
The shape of your pasta is crucial too. Whilst thick, ragù-style sauces are bold enough to stand up to big shapes like pappardelle and rigatoni, with pesto you want shapes like fusilli or trofie which have plentiful bends, grooves, twists and troughs. These provide your pesto with the maximum number of places to hide.
As for the sauce itself, well, pesto is naturally pretty good at clinging to pasta. The secret to making it even stickier is to mix it with a little bit of the water you cooked your pasta in. This water is bursting with starch and acts as an emulsifier and a thickener, giving you the best possible chance of serving a restaurant-quality dish.