Extra basil leaves or pine nuts can quickly thicken up pesto. Add the leaves or nuts to the mixture and pulse it in the food processor to blend. If you have to add significantly more basil leaves than your recipe originally called for, the flavor of your pesto may be affected, and you will need to add more Parmesan cheese, garlic and pine nuts to even it out. Add small amounts of each ingredient at a time and taste before adding more. Just don't add more olive oil, or you risk thinning out your newly thickened pesto.

If you're out of fresh basil or you don't want to risk adding too much more basil flavor to your pesto, add a small bunch of torn spinach leaves to the thin pesto and pulse it in the food processor. The spinach will thicken the pesto without adding a distinctive flavor. If you don't mind adding a peppery flair to your pesto, add arugula leaves to the mixture.

Toss a handful of breadcrumbs, croutons or torn bread pieces into the food processor and blend to thicken the pesto. Use unseasoned breadcrumbs or croutons, or you may alter the pesto's flavor with added seasonings.

Add canned cannellini beans or chickpeas to your pesto to thicken it and add a burst of protein. Pour the beans into a strainer and rinse them with cool water, letting them drain completely before adding them to the food process. Add approximately 1/4 cup at a time, blend and check the pesto's consistency before adding more.