Preheat the oven to 375°F and lightly grease 4–6 individual ramekins or oven-safe dishes.

Melt the butter in a large skillet over medium heat.

Add the chopped onion and cook for 5 minutes, until softened and translucent.

Add the minced garlic, sliced mushrooms, and thyme to the skillet. Cook for 8–10 minutes, stirring occasionally, until the mushrooms are tender and most of their liquid has evaporated.

Sprinkle the flour over the mushroom mixture and stir constantly for 1 minute.

Gradually pour in the vegetable broth and heavy cream, stirring continuously. Cook until the mixture thickens, about 3–4 minutes.

Remove from heat and stir in the Gruyère cheese until fully melted and smooth. Season with salt and pepper to taste.

Divide the creamy mushroom filling evenly among the prepared ramekins, filling them about 3/4 full.

Roll out the puff pastry slightly if needed. Cut circles or squares large enough to cover each ramekin with a 1/2-inch overhang.

Place the pastry over each filled ramekin, pressing the edges to seal against the rim.

Brush the top of each puff pastry with the beaten egg, and cut a small slit in the center to vent.

Place ramekins on a baking sheet and bake for 20–25 minutes, until the pastry is puffed and golden and the filling is bubbly.

Let cool for 5 minutes before serving.