Prepare the marinade for the steak by mixing together the balsamic vinegar, olive oil and rosemary. Place the steak in a large bowl and pour in the marinade ensuring all sides of the steak are coated. Cover tightly with cling film and refrigerate for at least 1 hour (maximum 24 hours).
Remove the steak from the refrigerator and bring to room temperature. Heat a grill pan or heavy-bottomed skillet over high heat. Remove the steaks from the marinade, and generously season with salt and pepper on both sides, using about one teaspoon of salt per pound of meat. Place the steak on the grill pan and cook until the meat is seared and deep brown on both sides (5 to 6 minutes per side for medium rare). Transfer the steak to a platter or cutting board and let rest for 5 minutes before cutting. Use a sharp knife and cut the steaks diagonally against the grain into ½ inch thick slices.
For the pumpkin cream
Preheat oven to 180°C. Using a sauce pan, heat the 100 g heavy cream with the cubed feta and the grated parmesan until the cheese melts and the mixture is completely combined. Remove from heat and allow to cool. Add the egg white, whipped cream, crumbled feta and pepper and mix until a mousse-like texture is formed. Refrigerate the cream to thicken.
Place the pumpkin in a baking dish, sprinkle with salt and pepper, drizzle with olive oil and bake in preheated oven for about 30 minutes, until very tender. When the pumpkin has cooled, mash with a fork, transfer to the cream mixture and gently stir.
Place salad leaves in a large bowl, sprinkle with salt and toss gently to distribute evenly. Drizzle the vinegar, olive oil and toss gently to coat the leaves. Place 4-5 slices of beef on each plate topped with 1 heaping tbsp of pumpkin cream. Add the salad leaves to the plate and serve.