Preheat the oven to 450°. Slice the squash crosswise 1 inch thick; scrape out any seeds. Lightly oil 2 baking sheets and arrange the squash slices on them. Season with salt and pepper and drizzle with 1/4 cup of the olive oil. Bake for about 25 minutes, or until just tender.
Combine the remaining 1/4 cup of olive oil with the vinegar, onion, sage, garlic and Sirach season with salt and pepper. Pour over the warm squash and let stand for 20 minutes.
Arrange the squash on plates, sprinkle with the basil leaves and serve( you can also throw this on a grill if you like for nice looking grill marks).