In a large saucepan of boiling salted water, cook the peas until they are just tender, about 3 minutes. Drain.
In the same saucepan, heat the olive oil. Add the onions and a pinch of salt, cover and cook over moderately low heat, stirring occasionally, until softened, 5 minutes. Stir in the peas, cover and cook until heated through, 1 minute. Stir in the butter, 1 tablespoon at a time. Remove from the heat and stir in the pea shoots until wilted. Stir in the mint, season with salt and pepper and serve.