With a food processor running, add garlic clove through the chute to mince. Scrape down sides of bowl and add spinach, basil, walnuts, cheese and salt; pulse until finely minced, scraping down sides of bowl as needed.
Turn food processor on and add oil through chute in a steady stream; process to your desired consistency. Taste for seasoning and add salt if needed. Use immediately or transfer to an airtight container and refrigerate for up to 1 week or freeze for up to 6 months. (See recipe notes for detailed storage instructions.)
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Notes
Store spinach pesto in the refrigerator by transferring to an airtight container and smoothing the top to an even layer. Then slowly add enough olive oil to cover the surface completely in a thin layer (this will prevent the pesto from turning brown). Cover and refrigerate for up to a week. Or freeze pesto for up to 6 months. For convenience, freeze in an ice cube tray then transfer frozen pesto cubes to a freezer-safe container or zip-top bag to use as desired.
If you like a garlicky pesto, add 2 or even 3 cloves of garlic.