Place peas and mushrooms on a piece of double-layer heavy-duty foil (about 18 in. square). Sprinkle with onions and dill; dot with butter. Fold foil around the mixture and seal tightly.
Grill, covered, over medium-hot heat for 5 minutes. Turn; grill 5-8 minutes longer or until the vegetables are tender. Open foil carefully to allow steam to escape. Season with salt and pepper. Yield: 8-10 servings.