Preheat oven to 400°F and line a baking sheet with parchment paper.

Melt butter in a large frying pan over medium heat.

Add mushrooms and diced onion. Sauté for 6-8 minutes, stirring occasionally, until tender and most of the liquid released by the mushrooms has cooked off.

Add garlic and fresh thyme. Cook for 1 minute.

Pour in white wine and simmer for another minute.

Stir in goat cheese and a splash of broth. Continue stirring until the cheese has melted and the filling comes together. Remove from heat.

Place thawed puff pastry on the prepared baking sheet. Use a fork to prick a few holes across the center of the pastry.

Spoon the mushroom filling into the center, spreading it out evenly while leaving a 1-2 inch border around the edges.

Fold the uncovered pastry edges inward over the outer edges of the filling, overlapping and pinching the corners to create a rustic tart.

Brush the exposed pastry with egg wash.

Bake for 25-30 minutes, or until the pastry is puffed and golden brown.

Let cool for a few minutes before slicing and serving.