Heat the olive oil in a heavy-based saucepan, add the chorizo and stir over a high heat for a few minutes until the oil has taken on a reddish-brown color from the chorizo. Add the onions, garlic and thyme and season lightly with salt and pepper. Cook, stirring occasionally for 2-3 minutes.
Pour in the butter beans and stir so that all the ingredients are mixed and coated in the chorizo oil. Add enough boiling water to just cover them. Bring to a simmer and cook gently for about 2 minutes
Add the cabbage to the pan. Stir in a little water if necessary so that there is enough liquid to cook the cabbage. Bring to a simmer and cook gently for about 5-7 minutes or until the cabbage is softened but still has a little crunch. When the cabbage is nearing the end of cooking, stir in the lemon zest and juice.
Scatter over the chopped parsley and cook for 30 seconds. Check the seasoning and adjust if necessary. Remove the thyme.
Ladle the soup into warm bowls and drizzle with olive oil (if desired for garnish) and some black pepper. Serve immediately with crusty bread.