I used this recipe for years in Australia, where it originates, with the green-skinned Kent pumpkin. So tasty for a low-cal recipe. I moved to the US and am still searching for a substitute as good as the Kent. Have tried butternut squash - wrong texture. I am now using canned pie pumpkin, which works quite well. Try adding fresh herbs or chile.
I made this today, after starting my new diet. Very nice. I didn't have any celery, so I substituted with a leek and a butternut squash for the pumpkin. After browning the onion as suggested, I chucked it all in the slow-cooker and it was a real delight. I estimated it at 1 WW per serving.