Clean and finely chop the onion, peel and cut the mushrooms into small pieces.
In a non-stick pan, heat the sunflower oil and fry the chopped onion over medium heat. Season with sea salt and sauté for about two minutes to soften.
Add the finely chopped mushrooms season with sea salt, freshly ground black pepper and dried marjoram. Sauté for about two to three minutes until removed from the water content.
Cut the meat to open like a book and place the loose tenderloin on a work surface lined with plastic wrap, cover it with the same plastic wrap and thin it with a meat tenderizer.
Spread the plastic wrap on the work table and place it over the slices of smoked bacon next to each other. Put over the tenderized turkey meat and season with sea salt and ground pepper.
Put the sautéed mushrooms on top of the meat, level it evenly, but leave a distance of one inch (two centimetres) from the edge; grate the cheddar cheese over the mushrooms.
With the help of the plastic foil, roll it lightly, pressing a little to tighten it until you reach the other end and put the roll with the opening underneath.
Grease the roasting tray with sunflower oil, place the stuffed turkey tenderloin in the tray with the opening down and season with freshly ground black pepper.
Add the water to the pan, cover with aluminium foil and place it in the preheated oven at 220 C (428 F) for one hour. Remove the foil and back for another 20-25 minutes.
Remove the turkey roll from the pan, let it cool a bit and then slice and serve. It can be served hot as a main course with mashed potatoes and tomato salad or cold as an appetizer.