Heat the butter in a large saucepan, add the shallot and soften for 5 minutes.
Add vinegar and sugar to the pan, then the beans, peas, most of the mint sprigs and 5 tbsp water. Bring to a simmer, cover and steam for 5 minutes, stirring once, until the beans are tender.
Remove the mint sprigs, turn the veg in the buttery juices and season to taste. Top with torn mint leaves.
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