Preheat the oven to 400°F Cut the Kabocha squash in half with a very sharp knife, scrape out the seeds and strings, and place the halves cut-side down on a lightly oiled non-stick baking sheet. Peel the turnips and cut them in wedges. Peel the celery root and cut it in 1” pieces. Toss the turnips and celery root with about half a tablespoon of the olive oil and a pinch of sea salt, and spread them on another baking sheet.
Roast all the prepared vegetables in the hot oven for about 50 to 60 minutes, or until the squash gives easily when poked with a wooden spoon, and the turnips and celery root are tender and flecked with dark brown (note: I found the celery root/turnips were done faster, at about 30 minutes Check on them periodically.) When the squash is cool enough to handle, scoop it out of its shell.
While those vegetables are roasting, cut the leeks in half lengthwise, wash them thoroughly and slice thinly, using only the white and very light green parts. Set aside. Chop the onion and sauté it gently in the remaining tablespoon of olive oil, with a dash of salt and the rosemary, until it is soft and golden brown.
In a soup pot, combine the roasted squash, turnips, celery root, leeks and sautéed onions with 4 cups water and a teaspoon of salt. Simmer the vegetables, covered, about twenty minutes to let them get perfectly soft. Add two cups vegetable broth, two tablespoons fresh lemon juice, a pinch of hot paprika or cayenne and 3 tablespoons pure maple syrup.
Allow the soup to cool somewhat, then puree it in a blender or in the pot with an immersion blender. Add a little more vegetable broth if the soup is too thick to pour easily from a ladle. Return the soup to a clean pot and bring it back to a simmer.
In a small saucepan, melt the butter. Lower the heat and keep cooking the butter for a few minutes, stirring with a whisk, until it is a light golden brown. Stir the browned butter into the soup.
Taste the soup, and correct the seasoning, whisking in more salt, lemon juice, or maple syrup as needed. This last step is essential, as Kabocha squashes can vary in sweetness, and lemons certainly vary in acidity. And as always, when working on the sweet-sour balance you reach that point where only a good pinch of salt will make it right.
Drizzle a thin thread of olive oil on top of each serving of this soup, and then sprinkle it with a spoonful of toasted, chopped pecans.