Ingredients:

6 tablespoons unsalted butter
1 large onion or 2 leeks (white and light green part only), chopped
2 to 3 celery stalks, diced
3 garlic cloves, finely chopped
3 rosemary or thyme branches
2 bay leaves
3 ½ pounds mixed root vegetables (carrot, parsnip, celery root, turnip, rutabaga, sweet or regular potato), peeled and cut into 1-inch chunks
2 teaspoons fine sea salt, more as needed
½ teaspoon black pepper, more as needed
Juice of 1/2 lemon, more for serving
Extra-virgin olive oil
Flaky sea salt
Crushed Aleppo, Urfa or other chile flakes, optional
Grated Parmesan or pecorino, optional

Instructions:

Melt butter in a large, heavy-bottomed pot. Stir in onion and celery. Cook, stirring occasionally, until vegetables are tender, about 10 minutes. Stir in garlic, rosemary and bay leaves; cook 1 minute more. Add root vegetables, 8 cups water, salt and pepper. Bring to a boil; reduce heat to medium and simmer, covered, until vegetables are tender, 30 to 40 minutes.
Remove and discard rosemary branches and bay leaves. Using an immersion blender, purée soup until smooth. (Alternatively, you can purée the soup in batches in a blender or food processor.) If the soup is too thick, add a little water. Season with lemon juice and more salt to taste.
To serve, ladle soup into bowls and top with a drizzle of olive oil, a few drops of lemon juice, flaky salt and crushed chile or grated cheese, if desired.