Add the spinach, olive oil, lemon juice, Parmesan, and seeds to a blender.: The initial wave of herb aroma will hit you first, a fresh, grassy scent that signals the spinach is prime. As the ingredients sit in the blender bowl, the contrast between the glossy olive oil and the dry seeds is apparent, and this is why layering matters for a more even puree. One trap is overcrowding the blender which can lead to uneven processing; work in batches if your container is small.
Blend very well until very smooth, stopping to scrape down the sides of the bowl or using the stick that came with the blender as needed. You can also do this in a food processor.: You will hear the motor change pitch as the mixture becomes more uniform, and the surface will move in a glossy vortex. The texture should go from coarse to creamy, and scraping ensures no leafy islands remain on the sides. If you do not scrape, pockets of unblended spinach can stay fibrous and impart a slightly bitter note.
Season to taste with salt if needed.: The machine will produce a slightly different texture, often a touch more rustic with tiny bits of seed or leaf. The sounds here are more intermittent, a pulsing thrum rather than a continuous roar, which helps control chunk size. A common error is over pulsing, which can leave a paste that is too dense, so pulse and check frequently for the texture you prefer.
Use with a pound of pasta, as a topping for pizza, or however you like!: At this point the full flavor profile is coming together and the lemon will shine through, so add salt in small increments and taste after each addition. Salt wakes up the savory notes in the Parmesan and harmonizes the oil and greens. Rushing this step or oversalting can mask the delicate freshness of the spinach , so be deliberate and conservative.
Use with a pound of pasta, as a topping for pizza, or however you like: The finished pesto should coat ingredients smoothly, clinging to pasta or spreading across a crust. When you toss it with hot pasta the aroma brightens and the sauce loosens slightly, making it ideal for warm applications as well as cold. One misstep is adding it to intensely hot items without a little cooling, which can dull the green color, so consider tempering the pesto on a warm dish rather than a steaming one to preserve vibrancy.