Cook the broad beans and peas in boiling, unsalted water for 2-3 minutes or until tender. Drain and refresh under running cold water. Mix together in a bowl with the butter beans.
Stir-fry the courgettes in 2 tbsp of the olive oil for five minutes, tip into the bean mixture and season well with salt and pepper. Add the thyme leaves.
Whisk the remaining olive oil, white wine vinegar, Dijon mustard and sugar together, and season with salt and pepper. Mix into the bean salad along with the crumbled feta cheese. Pile into a serving bowl and chill until required.