In a medium saucepan, bring salted water to a boil. Blanch the peas in the boiling water until they are just tender and bright green, about 2 minutes. Refresh immediately by plunging them into a bowl of cold water. Drain and reserve the peas.
In a small pan, heat 1 tablespoon of the oil over medium heat. Add the onion, and cook until soft, stirring occasionally, about 5 minutes.
Meanwhile, in a small bowl, whisk together the mustard, the remaining 2 tablespoons olive oil, vinegar and honey, and season with salt and black pepper, to taste.
In a large salad bowl, mix the peas, onions, pistachios, and pour over the dressing. Add the mint and snow pea leaves, and gently toss to coat with dressing. Top the salad with the feta cheese just before serving.