Start with roasting the Hubbard squash. Cut it vertically in half. Scoop out the seeds and place cut side down on a sheet pan. I put a piece of parchment paper down first for easy clean up. Put the squash in a 350º F/177º C preheated oven. Roast for at least 55 minutes or until the squash skin is tender to touch. Take out and let cool for at least 15 minutes. Scoop out the meat. This part could be done several days ahead of time. Squash meat freezes well. I like to weight it out and than put in a freezer bag with the weight marked on it. If you have leftover squash meat, I also like to use as a filling with spices mixed in for ravioli or tortellini.
Take the red onion, peel it, cut in half and cut into thin half moon slices. In a large pot, add the olive oil and butter. When the butter begins to sizzle add in the celery, carrot and red onion. Turn the heat down to medium. I add in a few pinches of salt and a crack of pepper from the grinder. I like to put a lid on the vegetables to help them sweat and caramelize. After 10 minutes add in the curry powder, turn the heat down to low. Put the lid back on and let the curry powder cook with the sweated vegetables for about 5-7 minutes.
While the curry powder works its magic with the vegetables, take the potatoes and peel them. Cut each potato into small dice and place them in enough water so they are covered. Take the lid off the pot, give a stir and add in the roasted squash meat. Give a stir and let cook with the vegetables for about 5 minutes with the lid back on the pot on low heat. Drain the water off the potatoes and than add these into the pot along with the 1600 ml of cold water. Add in a few more pinches of salt. Bring to a boil and turn down to a low simmer. Let this cook for at least 45-50 minutes or until the vegetables are easily mashed with a fork.
You can either use a wand to completely blend the soup or a blender. I like to wait until the soup is pretty cool before I use my blender. Check the soup for thickness. You may need to add a little more water if the soup is too thick. You do not want porridge. Check the soup for seasoning. If it needs a little more salt or pepper, add it. Don't be afraid of salt. When you think of the large pot of soup it is very different seasoning it than seasoning a salad for yourself or two scrambled eggs. Salt brings out the flavor of things. Add in the heavy cream. Serve the soup as an appetizer or you can enjoy as a main meal with a baguette and a green salad for dinner.