Heat a tablespoon of olive oil in a heavy-based frying pan over a medium-low heat, add the onion and cook for 10-15 minutes until soft and sweet but not coloured. Add the garlic and cook for another three to four minutes.
Add the tomatoes and butterbeans, stir and leave to warm a little, then take off the heat and transfer to a bowl.
Add eight tablespoons of olive oil, the vinegar, olives, capers, dill and a good couple of pinches of salt to the beans and mix well. Add the chopped sausage if using. Taste, adding more salt, oil, vinegar as needed, until balanced.
Crumbled the feta over, serve alongside flatbreads or chunks of bread for dabbing up juices left in the bowl