In a saute or deep frying pan on low heat add the oil and butter and stir gently until the butter melts.
Add mushrooms to the pan, and sprinkle with salt and pepper. Increase the heat to medium-high and cook, stirring occasionally, until golden, about 8 minutes. Add cabbage and leeks, and cook until they wilt and have begun to caramelize, about 5 minutes. Add a little water if starts to dry out.