Preheat oven to 210°C (425°F).

In a large skillet, heat olive oil and butter over medium high heat. Sauté the garlic until slightly golden. Add onion and cook until translucent. Add mushrooms and cook until almost dry.

Sprinkle mushrooms with rosemary and season generously with salt and pepper. Remove from heat and allow to cool. Roll out puff pastry into a rough square about 3 mm (1/8 inch) thick. Place pastry on a parchment covered baking sheet. Brush all the edges of the pastry with beaten egg white and fold them over to create a double thickness border, pressing firmly. Prick the pastry (but not the border) all over with a fork, about 20 times.

Reduce oven temperature to 200°C (400°F).

Spread the cooled mushroom mixture over the pricked surface of the pastry. Arrange Mozzarina slices evenly over the mushroom mixture. Arrange rosemary sprigs over the cheese and dust with paprika.

Cook the tart for 25 minutes or until pastry is golden and cheese is well browned around the edges. Remove from oven and cool for a few minutes.

Cut in four and serve.

Cremini (café) mushrooms are a good choice because they brown quickly without giving off too much liquid.