Preheat the oven to 200C/390F/Gas 6.
Roll up the beef fillets to form two cylinder shapes, then wrap them tightly in cling film to hold the shape. Place in the fridge for 1-2 hours.
Remove the beef from the fridge and take off the cling film, taking care to keep the beef in a cylinder.
Heat an ovenproof frying pan and add two tablespoons of the olive oil and add the garlic and herbs.
Season the beef with salt and freshly ground black pepper, then place in the hot pan of garlic and herbs and sear on the top and bottom, until coloured.
Transfer the pan to the oven and cook for 6-8 minutes, or until cooked to your liking. Remove from the oven and leave to rest, retaining the pan juices for later.
Meanwhile, for the mushrooms, peel the Portobello mushrooms and place them in a small roasting tin. Season with the cumin, salt and cracked white pepper, then place in the oven to cook for ten minutes.
Meanwhile, for the spinach, heat another frying pan and add one tablespoon of olive oil, the spinach and the vegetable stock, and cook until the spinach has just wilted. Season with salt and freshly ground black pepper.
For the tomatoes, heat a frying pan with the remaining one tablespoon of olive oil. Add the plum tomatoes and garlic and cook for 1-2 minutes. Season with salt and freshly ground black pepper.
Preheat the grill to high.
To serve, place one mushroom on each of two plates, topped with beef fillet, spinach, tomato and finishing with the gruyere. Place under the grill for 30 seconds until the cheese is just melted. Drizzle the juices from the steak pan around.