Pat dry the eye fillet and cut into 4cm cubes, approximately 50 grams each. Season with salt and pepper.
Heat a large frying pan over high heat. Add oil, and sear the beef quickly, for up to one minute, until browned on 2 sides. Do not overcook. Transfer to a plate and reserve.
Add the mushrooms to the pan and sauté on high heat for 5 minutes, stirring occasionally until they release all their liquid. They should resemble chocolate biscuit crumbs. Reduce the heat to medium and add the garlic and thyme. Fry for 2 minutes stirring until the garlic is softened. Transfer to a small bowl and stir in chicken liver paté until smooth.
Cut the puff pastry sheets in half and in half again to create 4 identical squares. Cut the prosciutto slices in half to make squarish pieces.
Place half a slice of prosciutto off-centre on one piece of pastry, allowing the prosciutto to hang over one edge of the pastry. Spread a teaspoon of mushroom paté in a thin layer over the prosciutto and flatten with the back of a spoon. Top with a cube of beef and fold the prosciutto over, pressing onto the beef. Fold the corners of the pastry into the centre. Press together firmly so the filling is tightly sealed. Place seam side down in a baking paper lined container. Repeat this step with the other pieces until complete. Optional: lightly score a patten over the tops of your mini beef wellingtons.
Cover and freeze overnight.
The next day, pre-heat your oven to 230°C fan.
Spray a rack with cooking spray and place on top of an oven tray. The rack is important so that the pastry bottoms do not become soggy. Once oven is up to temperature, remove the mini beef wellingtons from the freezer and place on sprayed rack. Working quickly so they don’t thaw, brush the tops with beaten egg yolk. Bake for 25 minutes at 230°C until the pastry is golden.
Rest for 15 minutes before slicing each mini beef wellington in half. Arrange on a platter, scatter with fresh thyme and serve with your favourite relish.