In a skillet, heat the olive oil and butter over medium heat. Add the minced shallot and crushed garlic, cooking until they soften.

Mix in the chopped mushrooms and thyme. Cook, stirring occasionally, until the mushrooms turn deep golden and dry.

Stir in the cream or mascarpone, then season with salt and pepper. Add the grated Gruyère cheese and mix well. Remove from heat and let the filling cool slightly.

Roll out the puff pastry sheet and place the mushroom filling down the center third of the pastry.

Cut the sides of the pastry into angled strips.

Fold the strips over the center filling to create a braid, tucking in the ends.

Brush the top of the braid with the egg wash made from the egg yolk and milk.

If desired, garnish with herbs or flaky salt.

Bake in a preheated oven at 200°C for 25–30 minutes, or until the pastry is deeply golden and puffed.

Allow it to rest slightly before slicing and serving.