Wash and string the snap peas. Cook in boiling salted water until tender, about 5 minutes. Drain. Heat butter and oil in a large frying pan over medium heat. Add the mushrooms and cook, stirring occasionally, until they’re soft and the liquid has evaporated, about 10 minutes. Add the peas, salt and pepper. Heat through for 2-3 minutes. Add fresh lemon juice just before serving.