Rub the squash generously with olive oil and season well with kosher salt. Pierce 3 holes in the top of the squash with a sharp object to allow the steam to vent when cooking. Place on a baking sheet and roast for about 1 hour, or until thoroughly soft and easily pierced with a fork. Remove from the oven.
When cool enough to handle, cut open the squash, carefully remove the seeds and discard. Using a large, flat spoon, scoop out all of the flesh and place in a bowl or container, discarding the remaining skin.
Cut the leeks lengthwise and julienne into straw-like strips, about 2 inches in length, and set aside. (This may be prepared in advance and kept in ice water.)
Heat the butter over medium heat in a large heavy saucepan and cook until the color reaches brown-butter stage. Add the fresh sage leaves and continue cooking until the sage becomes slightly crisped. Add the julienne of leeks and continue stirring until cooked, about 2 minutes. Add the 1 tablespoon of salt and ½ teaspoon of pepper and stir to blend.
Add the cooked squash and continue to stir over medium heat to mash the squash and to blend all of the ingredients together. (Some large pieces of squash may remain, or they can be mashed with a back of a spoon if a smoother consistency is desired.)
Spoon the squash into a large serving bowl and serve.