Preheat the oven to 425 ̊F.
Brush olive oil on the cut sides of the squash and place cut-side down on a large parchment lined sheet pan. Lightly brush the outside of the squash with olive oil.
Add the carrots, celery, and shallots to the sheet pan. Season with salt and pepper, add a splash of olive oil and toss to combine.
Cut the top end off the garlic, drizzle with oil. Wrap in foil and place on the sheet pan.
Roast for 45 minutes, or until the squash is soft and beginning to brown in spots. Halfway through, flip the carrots, shallots, and celery.
While the veggies cook, fry the sage. Heat olive oil in an 8.5” skillet over medium-low heat. Add 5-6 sage leaves at a time and cook until crisp, 30 – 60 seconds. Using tongs, remove and place the sage on a paper towel. Sprinkle with salt.
When cool enough to handle, scoop out the flesh of the squash into a 4-quart saucepan, add the carrots, celery, shallots, and squeeze the roasted garlic out of its skin into the pot (discard the skin).
Pour in the stock, sprinkle in the harissa, and pepper. Bring to a boil, then lower the heat and simmer for 10-15 minutes.
Using an immersion blender, purée the soup until smooth and silky. Alternatively, purée the hot soup in a blender in small batches at a time. Remove or loosen the lid of the blender, cover the lid with folded dishcloth and hold it when blending. Repeat with small batches until complete.
Garnish with a dollop of yogurt, pepita seeds, chili oil, fried sage leaves. Serve with crusty bread or baguette toast.