First, heat the oven to 425F. Peel the parsnips and carrots, cut the woody core out of any parsnip thicker than about 2 inches at the shoulder, and cut everything into 1-inch chunks.

Next, toss the chunks on a sheet pan with 2 tablespoons of the oil, the thyme sprigs, the red pepper flakes, 1 teaspoon of the salt and the black pepper, then spread them into a single layer with space between the pieces.

Then roast for 30 minutes, turning once halfway, until the cut faces are brown in patches and a knife goes through a parsnip with no resistance. Pull out the thyme stems and discard them.

After that, warm the last tablespoon of oil in a large pot over medium heat, cook the shallot for about 4 minutes until translucent, then add the garlic for 1 minute more without letting it color.

Add the roasted vegetables and 4 cups of broth, bring it to a simmer, and hold it there for 5 minutes so everything comes to temperature and the roasted flavor moves into the broth.

Blend for a full 2 minutes with an immersion blender, or in batches in a jug blender, adding more broth until the soup coats the back of a spoon and falls off slowly. Season with the remaining salt.

Whisk the yogurt, lemon zest and juice, ginger, salt, honey, cumin and pepper together in a small bowl until completely smooth. It should be thick enough to sit in a swirl rather than sink.

Finally, ladle the soup into bowls and spoon the lemon ginger cream over the top rather than stirring it into the pot, since yogurt breaks above about 175F. Finish with thyme leaves and a little more black pepper.