Step 1Heat the oven to 400 degrees. 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 nonstick baking sheet. Peel the turnips and cut them into wedges. Peel the celery root and cut it into 1-inch 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.
Step 2Roast all the prepared vegetables in the hot oven until the squash gives easily when poked with a wooden spoon, and the turnips and celery root are tender and flecked with dark brown, about 40 minutes to an hour. Remove the pans from the oven. When the squash is cool enough to handle, scoop it out of its shell.
Step 3While those vegetables are roasting, chop the leeks and onions: Cut the leeks in half lengthwise, wash them thoroughly and slice them crosswise very thinly (both white and light green parts); you should have about 1 1/2 cups. Set the leeks aside. Chop the onion and set it aside.
Step 4In a large soup pot, heat the remaining tablespoon of oil over medium heat. Add the onions and a dash of salt and the rosemary and cook, stirring now and then, until soft and golden brown, 10 to 12 minutes. Stir in the roasted squash, turnips, celery root and leeks with 4 cups water and a teaspoon salt. Simmer the vegetables, covered, about 20 minutes to let them get perfectly soft. Stir in 2 cups vegetable broth, 2 tablespoons fresh lemon juice, a pinch of hot paprika or cayenne, and the maple syrup. Remove from heat.
Step 5Allow the soup to cool somewhat, then puree it in a blender, in batches, or in the pot with an immersion blender. The various flavors in this soup are better when blended into one harmonious new flavor, but you can make the texture whatever you like. I prefer this as a silky-smooth soup, but you can stop at a rougher puree if you like. 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.
Step 6Meanwhile, in a small saucepan, melt the butter over medium heat. 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.
Step 7Taste 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 vary in sweetness and lemons certainly vary in acidity. As always, when working on the sweet-sour balance, you will reach a point where only a good pinch of salt makes it right.
Step 8Sprinkle each serving of this soup with a spoonful of toasted chopped pecans and a drizzle of olive oil if desired. This makes about 10 cups of soup.