My go-to fast day soup recipe:
Chop/cube the following for a soup base - quantities are approximate but fit my medium sized Le Creuset:
1-2 leeks
1-2 stalks celery
1.5cm chunk of ginger root peeled and chopped up finely
1 fennel
1/3rd butternut squash
1-2 carrots
I add the leeks first to a pan on medium heat with a scant amount of olive oil then add the rest as I chop, stirring occasionally. Then add enough water to cover with a generous teaspoon of marigold bouillon. Bring to boil then simmer until squash and carrots are soft (say 20 mins). This is my base for a good 4-6 portions. I quite often freeze some of this.
Things I might add to portions and heat up again depending what I've got in or how many calories I'm eating that day:
baby leaf spinach
green beans (chopped)
frozen soy beans
halved cherry tomatoes
coriander leaves (sometimes I put chopped stalks in soup base)
prawns
grated ginger
poach an egg in the soup
I usually serve it with a spoon of miso paste and a bit of spicy seasoning. I think the base works out less than 100 calories per bowl plus whatever was added. But I'm working from memory there so could be wrong. It's more of a stew than soup by the time I'm finished with it and a very satisfying meal on a low calorie day.