Heat the oil and butter in a heavy bottomed saucepan and sauté the onions, garlic, celery, pepper and potato for about 4 or 5 minutes, or until the vegetables start to look translucent and soft.
Turn the heat down to low, cover and leave to gently sweat down for 5 minutes, stirring occasionally to ensure they don’t stick.
Pour over the stock or water, bring to the boil, turn down to a gently simmer, cover and cook for another 10 minutes or until the potato is cooked.
Add the beans, sweetcorn and milk, including all of their tin liquids, the Thyme and a little seasoning. Bring to the boil once more, turn down and simmer for another couple of minutes, just to allow the flavours to amalgamate.
Taste for seasoning and adjust as necessary.
Add a swirl of cream if necessary and a sprinkling of grated Gruyere or Cheddar to finish. Serve with thickly buttered crusty bread.