Heat the butter and oil in a large, lidded frying pan over a medium heat and fry the spring onions for 5 mins, covered, turning occasionally until browned. Mix the stock with the mustard and pour this over the spring onions. Bring to a simmer, cover again and cook for 10 mins, then remove the lid and cook for a further 15 mins.

Scatter in the peas and cook for 5-10 mins until the peas are cooked, the spring onions tender and the stock has reduced. Spoon onto a serving plate and scatter over the mint, feta and lemon zest. Squeeze over the lemon juice and serve.