Preheat your oven to 200C (400F). Line a 20cm (8in) springform pan with baking paper. I line the sides as well because egg likes to stick!
Melt butter in a frying pan on a medium high heat. Slice mushrooms and add to the melted butter with the rosemary and cook, stirring every now and then. You want the mushrooms to brown and soften. Will take 10 minutes.
Lightly whisk eggs in a medium bowl. Add a little salt and then the cooked mushrooms and grated parmesan. Pour everything into the prepared pan.
Bake for 15-20 minutes or until the frittata is golden and puffed up a little like a souffle.
Slice and serve warm or at room temp with salad on the side.