Preheat the oven to 210°C (410 °F).
Wash and cut the butternut squash in half vertically. Remove the seeds using a spoon.
Place both halves on a baking tray lined with parchment paper or a silicone mat. Coat both pieces with 1 Tbsp of olive oil and season with salt and pepper to taste. Place the pieces face down and roast them for 30-40 minutes or until both halves are completely cooked through and the flesh is tender.
In a large pot, sauté the onions and celery in butter and 1 Tbsp of olive oil over medium-high heat for 3 to 5 minutes.
Remove pot from heat and add the rest of the ingredients.
Scoop out the flesh of the butternut squash into the pot. If you want to leave the skin, discard the stem and the tough piece on the bottom, cut into medium-sized chunks and add them to the pot.
Bring the soup mixture to a boil. Once boiling, reduce to low-medium heat and cover the pot. Let it cook for roughly 10 minutes.
Remove the pot from the heat and blend the soup until smooth and velvety.
Taste the soup to adjust the seasoning as needed.