anything with beans/lentils really. i like Pasulj, Turkish lentil soup, and ful medame. these are all easy to prepare, cheap, and filling. you can also deviate from standard recipes by throwing other veggies into the mix (within reason). they also work very well with some soft boiled eggs and hot sauce.

Not exactly soup, but congee (rice porridge) is great for filling your belly and it’s very easy to make.
I guess you could make it watery enough to pour it out of a thermos if necessary.

Gazpacho. Throw whatever veggies you have in a blender, season and bam - soup. Add a filler if it doesn’t keep you full. Beans, lentils, potatoes, rice are all good options.

I second the taco soup. I call it tortilla soup and do it a little differently, as they stated. It is really simple and really quite delicious.

I make it with a whole chicken which is fairly cheap, in a slow cooker, hard to mess up. But just the other night i made it with some plain chicken breasts i picked up that day. Butterfly cut them seared them off and finished them in the soup.

Three or four cups of Chicken stock.

I omit the taco seasoning and just use healthy amount of cumin and a little chili powder. Salt (depending on salt content of chicken stock), pepper, garlic powder.

Cans of corn, black beans, garbanzo beans, roasted tomato.

I recently started sauteeing a heap of mushrooms chopped to bits and adding that too.

Finish with a squirt of lime, some cheese, and tortilla chips, or i sometimes crisp up some regular corn tortillas and cut them up in it.

Super simple, tasty. I personally can eat it throughout the week.

Spanish lentils soup.

Fry some onion, garlic and chorizo (you can fry it directly in the pot with some olive oil).

Add water, lentils, potatoes, tomatoes, bay leaf, salt.

Cook.

Eat.

Delicious. It’s maybe 15 minutes of chopping and hour or two of cooking. If you have some pressure cooker it’s fast and easy. Without it it takes longer but it’s still really easy, just stir from time to time. Try it once to figure out the proportions but it’s hard to mess up.

Bean & vegetable soup. Easy to change up the beans used, and to vary the vegetables over the seasons, as well as whatever herbs & spices you like. Useful if the supermarket discounts things at the end of the day, or if they don’t always have everything in stock, as you can adapt it on the fly.

Scotch broth - mix of lentils & soup barley, usually with a small amount of carrot & cabbage, but you could use any vegetable. Useful if you forgot to start soaking your beans or pulses on time, as it barely requires soaking.

Alternatively you could make an extremely strong vegetable stock, freeze that as icecube-sized amounts, then pop a couple in a pan, add water to dilute to the strength of regular stock & simmer the rest of your ingredients in that. Making the stock would take a fair bit of time as you’d have to reduce it so much, but then you’d be set up for much shorter weekly soup-making sessions. Issue here is, if you’re not totally happy with a batch of stock, you’re stuck with it for a lot longer than a batch of soup.

I’m a fan of taco soup. Plenty of recipes out there to find one that works for you. The gist is some stock, beans, tomatoes, green peppers, ground beef, and taco seasoning mix. You can add some onions, or cheese, or sour cream, or crushed taco chips (stale works great), or some extra spice from peppers… basically anything goes.

Onion & carrots, anywhere from 1:2 to 1:1 by weight, boil in just enough water to cover, grate in some fresh ginger, salt, puree. Add more water if it’s too thick. Throw in some chicken bouillon if you like.

Could make pasta sauces, like bolognese, and freeze them, then make some pasta when you want it, like 9-8 minutes. Could also pre cook the pasta although I’ve never be a fan of reheated pasta. Hell, maybe could make a pasta dish and freeze the whole thing. Not exactly a soup, but i suspect pasta and pasta sauce evolved from soup.

I quite enjoy this one.

The sausage might not make it terribly cheap though.