I used sun-dried tomatoes packed in olive oil to make this. To make the pesto, you use the oil that the sun-dried tomatoes are packed in (hooray for using everything...), adding additional olive oil (if necessary) to bring the total amount to 1 cup (250ml.) That was the right amount of oil for me but since jarred tomatoes can vary, start with 3/4 cup (180ml) of oil, adding more until it's a nice, spreadable consistency. If you don't have a food processor you could make this in a mortar and pestle or a blender. I like to keep it a little on the chunky side so if using a blender, keep an eye on the texture. If you don't have walnuts, you can use almonds or another nut. Note: This recipe makes quite a bit. It's a good amount if you want to toss it with warm pasta for a main course, but you can cut it in half if just using it as a spread. Or you can make the full recipe and freeze half of it to use at a later date.