Heat a cast iron skillet over medium-high heat with 1 tablespoon oil. Pat beef cubes dry with paper towels, season with salt and pepper, then sear 1-2 minutes per side until browned but still raw inside. Transfer to a plate and cool completely.
8 oz beef tenderloin, salt, black pepper
Add butter to the same skillet over medium heat. Add minced garlic and cook 30 seconds until fragrant. Add finely chopped mushrooms, thyme, salt, and pepper. Cook 8-10 minutes, stirring occasionally, until all moisture evaporates and mushrooms form a thick paste. Cool completely.
8 oz mushrooms, 2 cloves garlic, 1 tbsp butter, 1 tsp fresh thyme leaves, salt, black pepper
Roll out thawed puff pastry and cut into 3-inch squares. Brush each beef cube with Dijon mustard, wrap with a half-slice of prosciutto, then spread 1 tablespoon mushroom duxelles on each pastry square. Place beef in the center and fold pastry corners over the top, pinching seams to seal. Brush all sides with beaten egg wash.
8 oz beef tenderloin, 4 slices prosciutto, 8 oz mushrooms, 1 sheet frozen puff pastry, 2 tbsp Dijon mustard, 1 egg egg
Preheat air fryer to 400°F for 3 minutes. Place bites seam-side down in the basket with 1-2 inches of space between each piece. Air fry 10-12 minutes until pastry is golden brown and puffed. Check internal beef temperature at 10 minutes aiming for 130-135°F for medium-rare.
Transfer bites to a wire rack and rest for 5 minutes before serving. This keeps the pastry crisp while beef juices redistribute. Serve immediately with dipping sauce on the side.
