Preheat the oven to 200°C.
Heat some oil in a large pan and quickly fry the seasoned beef all over until it's brown. Remove and allow to cool. The point of this is simply to sear the beef and seal all those juices in, you don't want to cook the meat at this stage. Allow to cool and brush generously with the mustard.
Roughly chop the mushrooms and blend in a food processor to form a puree. Scrape the mixture into a hot, dry pan and allow the water to evaporate. When sufficiently dry (the mixture should be sticking together easily), set aside and cool.
Roll out a generous length of cling film, lay out the four slices of Parma ham, each one slightly overlapping the last. With a pallet knife spread the mushroom mixture evenly over the ham.
Place the beef fillet in the middle and keeping a tight hold of the cling film from the outside edge, neatly roll the parma ham and mushrooms over the beef into a tight barrel shape. Twist the ends to secure the clingfilm. Refrigerate for 10 -15 minutes, this allows the Wellington to set and helps keep the shape.
Roll out the pastry quite thinly to a size which will cover your beef. Unwrap the meat from the cling film. Egg wash the edge of the pastry and place the beef in the middle. Roll up the pastry, cut any excess off the ends and fold neatly to the 'underside'. Turnover and egg wash over the top. Chill again to let the pastry cool, approximately 5 minutes. Egg wash again before baking at 200°C for 35 - 40 minutes. Rest 8 -10 minutes before slicing.
Par boil the potatoes in salted water. Quarter them and leave the skin on. Sauté in olive oil and butter with the garlic and thyme, until browned and cooked through. Season. Remove the thyme and garlic before serving.
Separate the outside leaves of the baby gem (leaving the smaller inner ones for salads) and very quickly sauté them in a pan of olive oil with a little salt and pepper - just enough to wilt them.
Serve hearty slices of the Wellington alongside the sautéed potatoes and wilted baby gems. A classic mustard vinaigrette makes a great dressing.