Finely chop the onion and sauté in a splash of olive oil with the diced chorizo until soft but not coloured. Add the garlic and cook until fragrant, a matter of a couple of minutes or so. Add the halved tomatoes and cook until they break down and release their juice.
Add the stock or water.
Finely slice the runner beans on the diagonal so you get nice long pieces. Add these to the pan, season and simmer until the beans are cooked.