Set aside 1 large or 2 small parsnips for the croutons. Coarsely chop the remaining parsnips.

Heat olive oil and 1 tablespoon butter in a soup pot or large saucepan over medium-high heat. Add onion and celery, then saute until tender and translucent, about 8 to 10 minutes. Add chopped parsnips, potato, thyme, coriander, if desired, and salt and pepper to taste. Saute, stirring a few times, until heated through, about 5 minutes. Add wine or vermouth, bring to a boil, and reduce by half, about 4 minutes. Add stock, cover partway, bring to a boil, and reduce to a simmer. Simmer until the parsnips and potatoes are fork tender, about 40 minutes; mash with a large spoon.

Let the chowder cool, uncovered, at least 10 minutes. Filling a blender no more than two-thirds full, puree the chowder in batches, then return to the soup pot.

To make the parsnip croutons, cut the reserved parsnip(s) into ¼-inch dice. Heat the remaining 2 tablespoons butter in a skillet over medium-high heat. When butter stops foaming, add diced parsnip. Season with salt, pepper, and a large pinch of sugar. Saute until parsnip is nicely browned. Transfer to a paper towel to drain, then set aside.

To serve, reheat the chowder, adding cream or milk to your liking. Just before serving, add a squeeze of fresh lemon juice. Serve the chowder in bowls, garnished with parsnip croutons and fresh thyme.