Roast the sausages at 200C/180C fan/gas 6 until golden brown, around 20-25 mins. Meanwhile, fry the chopped onion in the olive oil with some seasoning for around 5 mins. Add the anchovies and capers, then tip in the cannellini beans and 400ml water. Add the white wine and pesto and cook for 10 mins. Season, squeeze over some lemon juice and serve with the sausages.