In a hot pan, seal the beef fillet in some oil and butter for approximately 4 minutes on each side and rest on a draining tray or kitchen towel. In the same pan, sweat some onions and garlic and add the chopped peeled prawns and cook for approximately 2 minutes with some fresh thyme, salt and pepper. Set aside to cool down. In another pan, wilt the fresh spinach in garlic and butter, cook for 3 minutes stirring continuously, allow to cool and put on a kitchen cloth and squeeze out all the water from the spinach, this will prevent water dripping while cooking. Dust some flour on a working area and roll the puff pastry thin with a rolling pin. Brush the beef fillet with some English mustard, mix the spinach with the chopped prawns and place on the fillet, pressing firmly to form a good layer of mixture. Place the fillet on the puff pastry, roll it and seal all edges, egg wash and sprinkle some poppy seeds or sesame seeds. Place on a baking tray and bake for another 20 minutes 180 degrees oven for a medium result. Allow the meat to rest for 10 minutes on a kitchen towel before carving it. Cut the carrots and asparagus to the slant and blanche for 2 minutes in boiling salted water, refresh immediately in ice cold water. Blanche the potatoes for 5 minutes in the same water used for vegetables and then transfer all the vegetables in another frying pan with some butter and oil and cook until golden brown. Cut the mushrooms in small pieces and fry in some butter and oil with garlic and fresh thyme for approximately 4 minutes on high heat. Season to taste and assemble the dish to your liking.