In a skillet, heat a drizzle of olive oil over medium heat. Add the minced shallots and garlic, sautéing until fragrant.

Then, add the chopped mushrooms, cooking until most of the moisture has evaporated (about 5-7 minutes). Season with salt and pepper. Set aside to cool.

In the same skillet, add a bit more olive oil, gather the finely chopped beef tenderloin, and season with salt and pepper. Sear the beef over medium heat just until browned (not fully cooked).

Add the Dijon mustard to the beef and stir to combine. Mix in the cooled mushroom mixture, blending flavors beautifully.

Preheat your oven to 400°F (200°C). Roll out your thawed puff pastry on a floured surface. Cut the pastry into squares approximately 3 inches by 3 inches.

Place a tablespoon of the beef and mushroom mixture in the center of each puff pastry square. Fold the corners over to encase the filling and pinch to seal.

Brush the tops with the beaten egg yolk (this gives a stunning golden color).

Place the assembled bites on a parchment-lined baking sheet and bake for 15-20 minutes, or until the pastry is golden brown and puffed up.

Let them cool for a few minutes before serving. These are best enjoyed warm.