Thaw the frozen puff pastry overnight in the refrigerator
Preheat oven to 400°F
Unroll the pastry or if using a thick square block, roll the pastry out into a 10x10-inch (or 12x12) square. Score the pastry, about ½-inch in from the edge to create a small border around the entire perimeter of the pastry. Dock the inner part of the pastry with a fork.
Bake for 12-15 minutes or until the pastry is fully cooked and golden brown. Remove the pastry from the oven. If the middle has puffed up, use the back of a spoon to press it down. Set it aside.
In the meantime, place a pan over medium heat. Add the butter. Once melted add the shallot, rosemary and mushrooms. Cook, stirring occasionally for about 8 minutes until the mushrooms are soft and starting to brown.
Pour in the wine and scrape any browned bits off the bottom of the pan. Bring to a simmer and reduce the liquid until only a very small amount remains in the pan.
Add the green beans, cook for 2-3 minutes until just warmed through.
Add the cream. Simmer for 6-8 minutes until the sauce thickens up and the vegetables are well coated in the thick cream. Stir in the parmesan cheese, chives, nutmeg and cayenne. Remove from the heat to cool slightly.
Spoon the green bean mixture on to the middle of the tart. Spread it out evenly to the inside of the scored border.
Place it back in the oven to heat through for 3 minutes.
Remove from the oven. Top it with the crispy onions. Cut the tart into pieces and serve immediately.
