Preheat oven to 200C
Cut potatoes into small cubes, place in a pan of salted water, bring to the boil and simmer for 5 minutes, drain.
Fry the bacon until just browned and then put in a ovenproof dish.
Add mushrooms and fry for about 5 minutes then put in ovenproof dish.
Add onion to pan and cook until softened. Add garlic, cook for a minute then add creme fraiche. Bubble for a minute then add to the ovenproof dish.
Saute the potatoes until lightly browned.
Add the potatoes to the ovenproof dish and stir.
Add rosemary and cover with grated cheese.
Bake for 20 minutes.
I saute the potatoes at the same time as cooking the bacon so that it doesn't take as long to prepare.