Add the sun-dried tomatoes, spinach, basil, sunflower seeds, garlic, salt, and pepper to a food processor. Pulse until finely chopped.
With the food processor running, slowly pour in the olive oil and vinegar. Process until mostly smooth, scraping down the sides as needed.
Taste and adjust the salt. For a thinner pesto, add a little more olive oil.
Notes
If your sun-dried tomatoes are packed in oil, you can use that oil in place of some or all of the olive oil. If they are dry, soak them in hot water until tender, then drain before using.To freeze, portion the pesto into an ice cube tray, then move the cubes to a freezer bag for up to 3 months. Thaw what you need in the refrigerator.