Preheat the oven to 180C / 350F / gas mark 5
Wash, scrub and roughly chop the parsnips into 1 inch chunks. Peel the garlic and place in a roasting tin with the parsnip.
Drizzle with 1 tablespoon olive oil, season, sprinkle with thyme, place in the oven and cook for 15 - 20 minutes until tender.
Meanwhile, place the thinly sliced onions in a skillet with 2 tablespoons of oil and cook on low for 15-20 minutes, stirring occasionally, until softened and caramelised. Don't let the brown too much.
Drain and rinse the beans, set aside.
When the parsnip is tender and onion caramelised, add both to a blender together with the roasted garlic, beans, broth, miso paste and cayenne pepper, if using. Season well and blend until smooth. Add more broth or water if you want a thinner consistency.
You can use a stick blender for this step too, otherwise place soup in a large pot, ready to heat through.
For the croutons, cut the sourdough slices into ½ inch squares and place in a hot skillet. Toss generously olive oil, season with salt and toast for 5-7 minutes, until golden and crispy. Now, toss in finely chopped herbs.
Transfer to a bowl and leave to cool.
For the parsley oil, bring a pot of salted water to a boil and prepare a bowl filled with water and some ice cubes.
Once the water is boiling, add the parsley and cook for about 15 seconds; just until the stems are bright green. Quickly transfer to the ice bath and let sit for a couple of minutes to cool completely.
Transfer the parsley to a dry tea towel and dry, squeezing gently, until most of the moisture is out.
Transfer the parsley to a high-speed blender and add the oil. Puree until completely.
Pour the oil into a fine sieve and let sit. Do not press the oil through. Once done, transfer to a bottle and store in the refrigerator for a couple of days.
When you are ready to serve the soup, heat it through and stir some lemon juice through. Serve with croutons and parsley oil drizzled on top.