In a shallow microwavable dish toss the leeks in half the butter and cook on High for 5 mins until they begin to soften. Stir in the cabbage and mushrooms and add the crème fraîche. Lay the potato slices over the vegetables pressing them down with a fish slice.
Dot the potatoes with the remaining butter and microwave, uncovered, for 15-20 mins on High until they are done. Scatter over the cheese and thyme, and either microwave on High to melt for 2 mins, or grill until crisp and brown. Leave to stand for a few mins and serve straight from the dish.