Prepare the Beef Filling: Heat a large skillet, melt 2 tablespoons of butter with olive oil, sear beef pieces for 2-3 minutes, season with salt and pepper, and let rest.

Make the Duxelles: In the same skillet, cook finely chopped mushrooms and diced onion over medium heat for 5-7 minutes, add thyme and mustard, stir to combine.

Assemble the Bites: Roll out puff pastry, cut into squares, place duxelles and beef in center, fold and seal the pastry.

Chill for Efficiency: Place assembled bites on a baking sheet, cover with plastic wrap, and refrigerate for up to 24 hours.

Bake to Perfection: Preheat oven to 400°F (200°C), brush bites with beaten egg, bake for 15-20 minutes until golden brown.

Serve and Enjoy: Let cool slightly, garnish with sea salt and parsley, serve warm with horseradish cream sauce.