Is Pesto Halal?

Short answer
It is possible to find halal-certified pesto, but as a rule, no, pesto is not halal.

Long answer
Pork and alcohol are the two most cited foodstuffs that are prohibited in the Qur'an, so on the face of it, some people would be forgiven for assuming that pesto is permissible.

However, any cheese that has been produced using animal rennet is prohibited (haram). That is a problem because both Parmigiano-Reggiano and Pecorino (the two cheeses required in the traditional pesto recipe) are produced using it.

Vegetarian pesto or vegan pesto (in which the cheese is most commonly replaced by tofu or nutritional yeast) are the only pestos that you can safely say are halal.