In a dry frying pan on medium heat, stir and toast the walnuts for 3-5 minutes until toasted and slightly fragrant. Let them cool down to room temperature.
In a food processor combine the sun dried tomatoes and the oil they're in, the thyme, garlic, lemon zest and juice, parmesan and chili flakes. Blend everything together until it everything is finely chopped. .
While the blender is running, add in extra olive oil as needed until the pesto reaches your desired consistency. Taste and add any salt or lemon juice as needed.
Notes
The amount of oil will differ per jar, so the amount of extra olive oil needed will change too.