Look in the fridge and the veg rack. Prepare whatever is there, ensuring at least one onion. Sweat the onion in a large saucepan with some oil or fat. Add the rest of the veggies. Pour on some stock, or boiling water plus a stock cube. Add lentils and pearl barley, or rice, or spelt or whatever other grain. simmer as long as it takes (slow cooker is good). Taste. Add, if necessary, seasoning, worcestershire sauce, mushroom ketchup, whatever seems like a good idea to give body, bite or whatever.
As I live alone and soup making involves producing a lot of the stuff, I eat as is on day one, and keep the rest in the fridge. Day two, take out a portion and blend it. Day three, add some chopped up meaty stuff.
If replacing the lentils with any other pulse with a skin on it, apart from fresh peas, ensure the soup is boiled for at least ten minutes, or use a tin. It isn't just red kidney beans.