01 - Set the oven to 400°F to prepare for roasting the Jerusalem artichokes.
02 - Toss Jerusalem artichoke chunks with 1 tablespoon olive oil, a pinch of salt, and half the thyme. Arrange on a baking tray and roast for 25 to 30 minutes, turning once halfway, until golden and tender.
03 - Heat remaining olive oil and unsalted butter in a large pot over medium heat. Add diced onion, sliced leek, and minced garlic; cook for 5 to 7 minutes, stirring occasionally, until softened but not browned.
04 - Add the diced potato to the pot and cook for an additional 2 minutes, stirring to combine.
05 - Add roasted Jerusalem artichokes, remaining thyme, and vegetable stock to the pot. Bring to a boil, then reduce heat and simmer for 15 minutes until all vegetables are very tender.
06 - Remove pot from heat. Puree the soup until smooth using an immersion blender or, in batches, a countertop blender.
07 - Stir in heavy cream and lemon juice. Season with salt and black pepper to taste. Adjust consistency with additional stock or water if necessary.
08 - Reheat gently if needed, then ladle soup into bowls. Garnish with chopped herbs, a drizzle of olive oil, and optional croutons.