Preheat the oven to 400°F Brush the olive oil over the cut sides of the squash. Sprinkle with salt and pepper and place, cut-side down, in a glass baking dish. Bake for about 45 minutes, or until soft. Let cool. Scoop the squash out of the skin and puree in a blender or food processor until smooth.
In a large skillet, melt 1 tablespoon of the butter over medium-high heat and saute the leek until browned, about 7 minutes. Add the squash, cheese, and parsley and stir until just heated through.Season with salt and pepper to taste.
Place 1 tablespoon filling on a wonton wrapper. Brush the edges of the wrapper with water, fold, and seal shut around the filling. Repeat until all the filling is used.
In a large pot of salted slowly boiling water, cook the ravioli until they float to the top, about 4 minutes. Using a slotted spoon, transfer to serving bowls.
While the ravioli are cooking, melt the remaining 2 tablespoons butter in a small saucepan over medium heat. Brown the sage leaves in the butter for 3 to 4 minutes. Pour over the ravioli and serve at once.