Pat beef cubes dry with paper towels, then season them lightly with salt and pepper to build strong savory base flavor.

Heat oil in a pan and sear beef cubes briefly, browning sides while keeping centers pink and juicy for baking.

Remove beef onto a plate to cool, because hot meat will melt pastry early and ruin shape or flaky layers.

In the same pan cook butter, onion, garlic and mushrooms slowly, letting moisture evaporate to make thick flavorful filling.

Stir herbs and a little salt into mushrooms, then cool mixture fully so steam does not soften prepared puff pastry.

Cut puff pastry into small squares, large enough to hold beef cube and spoon of mushroom mixture comfortably inside.

Brush top of each beef cube lightly with Dijon mustard, adding gentle tang that balances rich meat and buttery pastry.

Place mushroom mixture on pastry square, set beef cube on top, then fold corners tightly sealing edges to prevent leaks.

Brush each wrapped bite with beaten egg wash for glossy golden color, then place on parchment-lined baking tray evenly spaced.

Bake until pastry is puffed and deeply golden, remove from oven, rest briefly, then serve warm, flaky Mini Beef Wellington Bites proudly.