What would be the easiest way to make soup?
The easiest soup is probably leek and potato.
It's basically a chopped leek and leftover mashed potato, boil for 5 or 10 minutes till it looks right, and eat.
You can use chopped fresh potato instead of mash (adjusting the cooking time), or add more to it - I prefer vegetable stock, but chicken or ham stock is good too. Smoked garlic is wonderful. Herbs like parsley, dill, oregano, or spices like smoked paprika or mustard. A strong grated cheese added at the end is amazing. Finely chopped onions or shallots, sprinkled in the bowls just before serving. Milk, cream (fresh or soured), plain yogurt or buttermilk can go in too, if not already in the mash, along with a huge knob of butter - I recommend Cuinneog Fermented Irish Farmhouse butter, which is so delicious you could eat it out of the packaging with a spoon.