The simplest vegetarian soup: zucchini soup.
Caramelize one chopped yellow onion and some garlic in a pot. Cut enough zucchini into half moons to fill your pot 3/4 up and throw them in with the onions and garlic. Pour in vegetable stock to cover, then simmer until zucchini is tender. Blend.
Serve with good crusty bread and a dollop of sour cream or bit of crumbled Stilton.
Sometimes I add a few handfuls of kale before blending for extra goodness.